User Exit COOM0001 User Exits for Overhead Surcharges - How to use it

Hello ABAPers,

I am a PLM consultant and on my service order costing sheet, I want to use COOM0001 user exit to calculate surcharges/ credits on my base value. Can anyone help me with understanding where does it gets invoked/trigger. The only place I have seen upto now is when I run KGI2 tcode, overhead activity allocation. Can it also be triggered from within the IW31 or IW32 tcodes. The second question is that has anyone implemented this user exit where it looks at the cost element associated with each postings on a work order and then determine what kind of surcharges to be applied.

Any help would be appreciated

thanks

Qazi