A surety that writes your performance and payment bonds is effectively guaranteeing your work. Bankers extending a line of credit are lending against the same balance sheet. Both read your WIP schedule closely, because it is where the income statement's estimates live.
They tend to look for the same things: under billings that might hide losses or uncollectible change work, fade patterns across jobs, gross profit left in backlog, over billings versus cash on hand (over billing is fine if the cash is actually there), and whether the estimates on this year's WIP held up compared with last year's.
Bonding capacity is often discussed with rough multiples of working capital and net worth, and a single-job limit that is a share of the aggregate program. These are rules of thumb, not formulas: each surety underwrites differently and also weighs experience, character and the type of work.
The practical lesson: a WIP that is timely, consistent and honest is itself a credit asset. Surprises are what cost you capacity.