Standard programs
RPTPSH10 : Personal work schedule, also accessed via PA20/PA30 infotype 2001.
User Exits in CATS
The user exits are available using transaction SMOD. Custom code can be put inside them to customize the CATS behaviour.
http://wiki.sdn.sap.com/wiki/display/ERPHCM/User-exits+in+CATS
Confuguring and Using CATS
Excerpts from SAP Press Book 'Confuguring and Using CATS'