How to Credit a invoice line having revenue schedule using unit revenue reversal rule

We are trying to use 'Unit' revenue reversal rule for crediting a invoice line with revenue schedule, below are the invoice and credit memo details.

Invoice

1. Invoice Rule: In Advance
2. Invoice amt: 600
3. Revenue Schedule: 6 Month Fixed, Start date: 1st Dec 2020
4. qty: 6
5. unit price: 100

Credit Memo:

1. Revenue reversal Rule: Unit
2. Credit Memo Date: 1st Feb 2021
3. Credit Amt: 300
4. number of Units: 3(for example)
5. Last Period to credit: We are unable to rationalize or understand how this period will change the accounting or how credit is applied e.t.c.

Thanks and Regards