ZigBee structured attributes and their content type


I am not sure if this is the right place for the following question, since it involves the generic ZigBee Cluster Library specification.

Anyway, my problem is how to get the data type for the elements of structured (i.e., Array, Structure, Set, Bag) attributes. Specification at hand, it seems there is no hint as to where such information is held.
This puzzles me a bit, since you clearly cannot expose, let’s say, an Array attribute without specifying the size of each of its elements. If, for example, you read the specification for Read/Write Attributes Structured Command, the content type information is lacking and therefore seems to be agreed beforehand.
Where’s the catch?