You are a functional consultant configuring items in Dynamics 365 Business Central.

In Business Central, the Item Units of Measure page defines conversion rules between the base unit and alternate units.

The field Qty. per Unit of Measure specifies how many base units are contained in the alternate unit.

Since the base unit is PCS, and one BOX contains 20 PCS, the correct configuration is:

BOX → Qty. per Unit of Measure = 20.

The system then automatically converts between PCS and BOX when documents are created.

Setting PCS to 0.05 or 20 would be incorrect because PCS is already the base unit (it always has 1 as Qty. per Unit of Measure).