In article <doit.wisc.edu>,
Charles Thomas <facstaff.wisc.edu> wrote:
It's a field in a structure (eg in a WindowRecord) that's usually for
the programmer to use (for instance, to store a pointer to some data
related to the window). IIRC, in some cases this was also used to
communicate with the operating system.
It's probably described in Macintosh Toolbox Essentials :
Patrick Stadelmann <ch>