User exit name and the correct internal table/trans table

Hello All, I need to place a billing block on Sales order if the pricing condition of any of the order items is not a contract price (checking for pricing condition ZR00 in my case)

What is the internal table or transparent table that need to looped through to check the pricing conditions for each item in the order?

Is "FORM USEREXIT_MOVE_FIELD_TO_VBAK" in the include MV45AFZZ right place to do so? or is it at FORM USEREXIT_SAVE_DOCUMENT in the sae include program?

Thanking you in advance.