True "severance" pay is never Plan Compensation and is different from statutory post-severance compensation. If a person receives a "separation bonus" as part of their last paycheck, that may be includable depending on the terms of the plan - but if it's classified as severance it is ignored in any scenario.
If this is includable post-severance compensation, and is not known until after the ASD, often a person is put into pay status based on the known (but incomplete) compensation amount and then after a final accurate benefit is calculated the future benefit payments are adjusted along with a true-up payment. Or, forms and elections are made using the aforementioned estimate prior to the ASD but then commencement is delayed for administrative reasons as the final actual benefit gets calculated and then paid effective back to the ASD. This is NOT an RASD if the QJSA forms are provided to the participant prior to the ASD. Plan language must specifically provide for an RASD and there are certain rules you must follow in calculating and communicating benefits.