There isn't a rule to force you to fill in the Payment Company on the Placement.
What the system will do is if there is an Payment Company on the placement then it will use that, if one is not filled in then it will use the one set on the applicant.
You would get an error is say the applicant had PAYE details and the placement is set as Ltd/Umbrella.
Then as the placement has exported with that data it won't let you change it
