org-agenda-cmp-user-defined

org-agenda-cmp-user-defined is a variable defined in `org-agenda.el'.
Its value is

nil

Documentation:
A function to define the comparison `user-defined'.
This function must receive two arguments, agenda entry a and b.
If a>b, return +1. If a the user comparison, return nil.
When this is defined, you can make `user-defined-up' and `user-defined-down'
part of an agenda sorting strategy.

You can customize this variable.