What is a custom object? A custom object is a piece of shared code that gets loaded at runtime into the core of Ayam to allow modeling with new types of geometric objects (a plug-in-system for object types).
You should take a look at (or print out) the files
"plugins/csphere.c" and
"plugins/csphere.tcl" from the src directory of the
Ayam distribution.
They contain a more or less documented example custom object that
does nothing exciting, but implements a simple sphere.
Additionally, they may serve as starting point for your
own custom object. Just copy the files and rename all
occurences of the string "csphere" to your
own new name.
Take a look at the modules
"extrude.c", and "icurve.c" in the objects
sub directory of the sources. They implement objects
too, but support some additional advanced functionality:
notification (extrude), single point editing (icurve),
conversion (extrude, icurve), and provide mechanism (icurve).