MIGO Is mandatery in Credit memo in 3rd party ?

Dear  SD Experts ,

i know the 3rd party processing But i have One Doubt please explain me

2 Steps we have to do In Third part return process.

link tells

http://scn.sap.com/thread/987949

1 Flow  to bring the stock from Customer is

va01 return order ( W/O Ref) --vl01n- Ret deli---MIGO ---VF01 ( Credit memo with ref to Return delivery )

2 Flow  to Send the stock from company to vendor

ME21N( Ret PO )-- MIGO---MIRO ( Credit memo with ref to Ret po )

i know this flow is Right to credit memo in 3 Rd party But in both cases MIGO IS Mandatory or not ?

Second thing is TASG Is item category For credit memo in third party ..i have checked in VOV7

But i didnt understand this where i will insert /

Thanks a lot