transaction Key PRD_PRA

HI All,

In which scenario OBYC Settings Transaction key PRD--PRA  will be used. we did nt activated ML. Still the system is throwing a message at  that time of cancellation of production order confirmation "Account determination for entry PRD_PRA  valuation class XXXX not possible.

1.For PRD_PRA-- ML Need to activated or not??

2. how PRD-PRA is differ from the PRD

3. in which scenario PRD_PRA  will be assigned with GL

Thanks