Data often needs to be grouped together, for example, consider a list of
icons. Sometimes a trick like arranging little images into a big raster
works, but generally they'll need to be structured as a first class group.
The objects "LIST" and "CAT " are IFF-universal mechanisms for this
purpose.  Note: LIST and CAT are advanced topics the first time reader
will want to skip.

Property settings sometimes need to be shared over a list of similar
objects. E.g., a list of icons may share one color map.  LIST provides a
means called "PROP" to do this.  One purpose of a LIST is to define the
scope of a PROP. A "CAT ", on the other hand, is simply a concatenation of
objects.

Simpler programs may skip LISTs and PROPs altogether and just handle FORMs
and CAT s.  All "fully-conforming" IFF programs also know about "CAT ",
"LIST", and "PROP".  Any program that reads a FORM inside a LIST must
process shared PROPs to correctly interpret that FORM.

 Group CAT     Group LIST     Group PROP     Properties for LIST