Can I specify a purchase ('P') item as a secondary output on a BOM?
DBA does not allow a purchase item to be a secondary output for a BOM. It introduces technical problems with the Cost Rollup. We do intend to allow this in the future, once we solve the problem it creates with the Cost Rollup, but there is currently no estimated timeframe for that change (it's a large structural change). Currently, you can list a purchase item as a secondary output for a Job but not a BOM (because Jobs aren't considered with Cost Rollup). So, if you must have a secondary output of a purchase item, then that will need to be specified once the Job has been created.
You may also consider using a "place-holder" secondary output in the BOM (which would, again, need to be a manufactured item) and then switch that place-holder with the purchase item at Job creation.