Electrical panel bus rating
That's a Square D panel and in their world, a 12 space - 24 circuit panel is rated 125A. The largest 100A panel they sold was 8 space - 16 circuit. This is what they offer CURRENTLY, but in reality not much has changed in the last 30 odd years with them.

But that doesn't mean your SERVICE is rated for 125A, that would be determined by the main breaker that is feeding this panel. You will need that information as well. Here is the rule they are referring to:

(Busbar Rating (A) x 1.2) - Main Breaker Rating (A) = Max PV (A)

Most likely the solar contract already knows the size of your main, but he doesn't want to take on responsibility for determining the size of your panel bus. So if your panel bus is 125A and your main Service breaker is 100A, then your PV system limit is (125 x 1.2) - 100A = 50A of PV solar. But if your panel bus is only 100A and your breaker is 100A, your PV limit is only 20A.