Progress billing is invoicing a customer at set intervals during a job for the work completed so far, instead of once at the end.
It's standard on commercial and public work that runs more than a few weeks, where you're buying hundreds of tons of mix and paying crews long before the job is done. Each period, usually monthly, you submit a pay app showing how much of each line on the schedule of values is complete. The owner or GC approves what they agree is done and pays it, less retainage.
Signed change orders go onto the schedule of values as new lines, so they get billed as the work is done.
Residential and small commercial jobs usually skip all this and bill a deposit and a final invoice.
In Pavr the schedule of values comes off the signed proposal line by line, each pay app bills the percent complete on each line, and signed change orders land on it as their own lines.