How PAB is calculated in S&OP constrained plan

How PAB is calculated in S&OP constrained plan

We understand PAB in Supply plan is calculated as Total Supply- Total Demand. Where total supply incudes onhand, intransit supplies, PO, work orders, etc.

And Total Demand includes - Sales order, Forecast, Dependent Demand.

But How S&OP constrained plan is calculating PAB. Which all measures it includes while calculating. We noticed that its not calculated the same way as it does in Supply Plans.

Also, we observed that PAB never goes Negative in S&OP.

Thanks

Avinash