P4210 - Why do I not get a hard error if I try to enter an order line for a quantity less than the P

I have an item for which the Primary UOM is Case.

But I find I am able to enter an order line for 0.5 of a Case and can submit and re-enquire on the order line.

Surely, wouldn't I have expected a hard error to prevent me from entering an order line for a quantity less than the Primary UOM?

I've done a bit of KM searching on this. Ref E1: 41: Unit of Measure Conversion (P41002/P41003) (Doc ID 2102783.2)

Shouldn't I expect something like a 085D error?

Error 085D – Issued when a valid conversion from the specified unit of measure to the primary unit of measure cannot be found. If using Transportation, make sure that conversions exist for the Weight and Volume units of measure to the primary unit of measure.