Some API information contains a list where each entry in the list is itself a data structure.
A good example is the Retrieve System Status (QWCRSSTS) API. It supports multiple formats for different types of information. The SSTS0300 format contains a list where each entry in the list has the information about a particular storage pool. In addition to the two critical fields (the offset to where the list begins field and the number of entries in the list field), the format also supports a field that describes the length of each entry. In the initial library list, each entry was 11-bytes long. But in a storage pool, a field (length of pool information entry) describes the length and should be used instead of a fixed-length increment. This allows for growth, such as more information being available in another release for each list entry.
For example, if another field is added to describe some additional information about a storage pool, it is probably added after the paging option field. The length of pool information entry allows your code to be upwardly compatible while it retains the locations (relative to the start of a list entry) of the current fields.