Complex sentence: The composite datatypes can be subindexed with an 8-bit index; the value in subindex 0 of an array or record indicates the number of elements in the data structure, and is of type UNSIGNED8.
Split into simpler sentences: The composite datatypes can be subindexed with an 8-bit index. The value in subindex 0 of an array or record indicates the number of elements in the data structure, and is of type UNSIGNED8.