Aggregate types are types such as arrays, strings and structures. The internal structure of an aggregate type is not transparent in the way that immediate types are. For example, two structures may have the same size of 8 bytes, but one might partition its bytes into two integers, whereas the other might be partitioned into a byte, an integer, and another byte. The FLI provides a number of functions to manipulate aggregate types. A feature of aggregate types is that they are usually accessed through the use of pointers, rather than directly.
LispWorks Foreign Language Interface User Guide and Reference Manual - 7 Dec 2011