Every progress billing starts with the schedule of values (SOV): the contract sum broken into line items, each with a scheduled value. The lines add up exactly to the contract sum. They are often organized by trade or by CSI division, and they are agreed with the owner or architect before the first pay app goes out.
The SOV matters because every later bill is measured against it. Each month you report how much of each line is complete, and that percentage of the line's scheduled value becomes billable. A good SOV has lines small enough that progress is easy to verify in the field, and large enough that the continuation sheet stays readable.
The classic temptation is front-loading: putting extra value on early work (mobilization, sitework, structure) to collect cash sooner. Reviewers know the pattern and push back, and if you get away with it you are over billed for most of the job, which shows up on your WIP and makes the back half of the job cash-poor.
When change orders are approved, they get their own SOV lines. Keeping them separate makes it easy to show the owner exactly what was billed against each change.