Complex sentence: Subtyping enables a given type to be substituted for another type or abstraction, and is said to establish an is-a relationship between the subtype and some existing abstraction, either implicitly or explicitly, depending on language support.
Split into simpler sentences: Subtyping enables a given type to be substituted for another type or abstraction. Subtyping is said to establish an is-a relationship between the subtype and some existing abstraction, either implicitly or explicitly, depending on language support.