Significance of P_UNITS_ASSIGNED_TBL parameter in request payload of Asset Transfer REST API

I want to understand the significance of P_UNITS_ASSIGNED_TBL parameter in the request payload of Fixed Asset Transfer through Fusion REST API

REST API Endpoint: https://domain-name/fscmRestApi/resources/11.13.18.05/erpintegrations

Scenario 1: I need to Transfer 20 units of Asset 100 from one distribution ID (4003) to a new one (-1). The distribution ID 4003 has only 20 units before transfer i.e., I am moving all the units to a new distribution ID. The new distribution is having same expense account as the current, but a different location. I have given P_TRANSACTION_UNITS_TBL: '-20,20' as I want to transfer 20 units.