Question: What does the Buddhist belief that says Buddhas come one at a time and not within other eras mean?

full_context: Document 1: Buddhism: The Buddha's death is seen as an illusion, he is living on in other planes of existence, and monks are therefore permitted to offer "new truths" based on his input. Mahayana also differs from Theravada in its concept of śūnyatā (that ultimately nothing has existence), and in its belief in bodhisattvas (enlightened people who vow to continue being reborn until all beings can be enlightened).
Document 2: Buddhism: There is no evidence that Mahāyāna ever referred to a separate formal school or sect of Buddhism, but rather that it existed as a certain set of ideals, and later doctrines, for bodhisattvas. Initially it was known as Bodhisattvayāna (the "Vehicle of the Bodhisattvas"). Paul Williams has also noted that the Mahāyāna never had nor ever attempted to have a separate Vinaya or ordination lineage from the early schools of Buddhism, and therefore each bhikṣu or bhikṣuṇī adhering to the Mahāyāna formally belonged to an early school. This continues today with the Dharmaguptaka ordination lineage in East Asia, and the Mūlasarvāstivāda ordination lineage in Tibetan Buddhism. Therefore Mahāyāna was never a separate rival sect of the early schools. From Chinese monks visiting India, we now know that both Mahāyāna and non-Mahāyāna monks in India often lived in the same monasteries side by side.
Document 3: Buddhism: Formal membership varies between communities, but basic lay adherence is often defined in terms of a traditional formula in which the practitioner takes refuge in The Three Jewels: the Buddha, the Dharma (the teachings of the Buddha), and the Sangha (the Buddhist community). At the present time, the teachings of all three branches of Buddhism have spread throughout the world, and Buddhist texts are increasingly translated into local languages. While in the West Buddhism is often seen as exotic and progressive, in the East it is regarded as familiar and traditional. Buddhists in Asia are frequently well organized and well funded. In countries such as Cambodia and Bhutan, it is recognized as the state religion and receives government support. Modern influences increasingly lead to new forms of Buddhism that significantly depart from traditional beliefs and practices.
Document 4: Buddhism: Approximately six hundred Mahayana sutras have survived in Sanskrit or in Chinese or Tibetan translations. In addition, East Asian Buddhism recognizes some sutras regarded by scholars as of Chinese rather than Indian origin.
Document 5: Buddhism: According to Mahayana tradition, the Mahayana sutras were transmitted in secret, came from other Buddhas or Bodhisattvas, or were preserved in non-human worlds because human beings at the time could not understand them:
Document 6: Buddhism by country: China is the country with the largest population of Buddhists, approximately 244 million or 18.2% of its total population. They are mostly followers of Chinese schools of Mahayana, making this the largest body of Buddhist traditions. Mahayana, also practiced in broader East Asia, is followed by over half of world Buddhists.
Document 7: Buddhism: Buddhists do not consider Siddhartha Gautama to have been the only Buddha. The Pali Canon refers to many previous ones (see List of the 28 Buddhas), while the Mahayana tradition additionally has many Buddhas of celestial, rather than historical, origin (see Amitabha or Vairocana as examples, for lists of many thousands Buddha names see Taishō Shinshū Daizōkyō numbers 439–448). A common Theravada and Mahayana Buddhist belief is that the next Buddha will be one named Maitreya (Pali: Metteyya).
Document 8: Buddhism: According to a demographic analysis reported by Peter Harvey (2013): Mahayana has 360 million adherents; Theravada has 150 million adherents; and Vajrayana has 18,2 million adherents. Seven million additional Buddhists are found outside of Asia.
Document 9: Buddhism: Buddhism /ˈbudɪzəm/ is a nontheistic religion[note 1] or philosophy (Sanskrit: धर्म dharma; Pali: धम्म dhamma) that encompasses a variety of traditions, beliefs and spiritual practices largely based on teachings attributed to Gautama Buddha, commonly known as the Buddha ("the awakened one"). According to Buddhist tradition, the Buddha lived and taught in the eastern part of the Indian subcontinent, present-day Nepal sometime between the 6th and 4th centuries BCE.[note 1] He is recognized by Buddhists as an awakened or enlightened teacher who shared his insights to help sentient beings end their suffering through the elimination of ignorance and craving. Buddhists believe that this is accomplished through the direct understanding and perception of dependent origination and the Four Noble Truths.
Document 10: Buddhism: During the period of Late Mahayana Buddhism, four major types of thought developed: Madhyamaka, Yogacara, Tathagatagarbha, and Buddhist Logic as the last and most recent. In India, the two main philosophical schools of the Mahayana were the Madhyamaka and the later Yogacara. According to Dan Lusthaus, Madhyamaka and Yogacara have a great deal in common, and the commonality stems from early Buddhism. There were no great Indian teachers associated with tathagatagarbha thought.
Document 11: Buddhism: In addition, Mahayana Buddhists believe there are innumerable other Buddhas in other universes. A Theravada commentary says that Buddhas arise one at a time in this world element, and not at all in others. The understandings of this matter reflect widely differing interpretations of basic terms, such as "world realm", between the various schools of Buddhism.
Document 12: Buddhism: The Mahayana sutras often claim to articulate the Buddha's deeper, more advanced doctrines, reserved for those who follow the bodhisattva path. That path is explained as being built upon the motivation to liberate all living beings from unhappiness. Hence the name Mahāyāna (lit., the Great Vehicle).
Document 13: Buddhism: In the Mahayana, the Buddha tends not to be viewed as merely human, but as the earthly projection of a beginningless and endless, omnipresent being (see Dharmakaya) beyond the range and reach of thought. Moreover, in certain Mahayana sutras, the Buddha, Dharma and Sangha are viewed essentially as One: all three are seen as the eternal Buddha himself.
Document 14: Buddhism: Two major extant branches of Buddhism are generally recognized by scholars: Theravada ("The School of the Elders") and Mahayana ("The Great Vehicle"). Vajrayana, a body of teachings attributed to Indian siddhas, may be viewed as a third branch or merely a part of Mahayana. Theravada has a widespread following in Sri Lanka and Southeast Asia. Mahayana which includes the traditions of Pure Land, Zen, Nichiren Buddhism, Shingon, and Tiantai (Tendai) is found throughout East Asia. Tibetan Buddhism, which preserves the Vajrayana teachings of eighth century India, is practiced in regions surrounding the Himalayas, Mongolia and Kalmykia. Buddhists number between an estimated 488 million[web 1] and 535 million, making it one of the world's major religions.
Document 15: Buddhism: China is the country with the largest population of Buddhists, approximately 244 million or 18.2% of its total population.[web 1] They are mostly followers of Chinese schools of Mahayana, making this the largest body of Buddhist traditions. Mahayana, also practiced in broader East Asia, is followed by over half of world Buddhists.[web 1]
ground_truth: The School of the Elders
sub_queries: What buddhist belief says that Buddhas come one at a time and not within other eras? Answer: Theravada
Theravada means what? Answer: The School of the Elders