Otherwise they can be registered only on the supplemental register. Secondary meaning is a mental association in the minds of buyers between the mark and the source of the product. Generally this is proved by surveys. Roughly speaking, arbitrary and fanciful marks are the strongest, then suggestive marks, then descriptive marks. Arbitrary marks use words, symbols, pictures etc.
Incongruity is one strong indicator of suggestiveness.
Descriptive marks describe the intended purpose, function or use of the goods, the size of the goods, the users of the good, desirable characteristics of the goods, or the end effect upon the user. Le Croissant Shop for food services. Obviously, there is a significant subjective component to these categories. Obviously, there is no likelihood of confusion if the two marks are used in completely different commercial fields. Actual confusion by customers is the best proof but is not required.