Fit-up
Holding the pieces of a fabricated assembly in the right relationship while somebody welds them, and usually the largest uncosted line on the quote.
Fit-up is the stage between cutting the pieces and welding them, and on most weldment quotes it is estimated by feel or not at all. It covers locating every member, squaring the assembly, clamping or fixturing it, and tacking it so the geometry survives the heat that follows.
The reason it is expensive is that it scales with piece count and complexity rather than with weld length. A frame of 14 members has 14 things to locate and hold, and none of that appears anywhere on a weld symbol, which is what estimators tend to count.
Something has to do the holding. On a one-off it is a welder with clamps and a surface table, and the cost sits in the hours. On a repeat program it is a built fixture, which is real money spent before the first piece and spread across whatever volume actually arrives.
The quoting error is assuming a fixture exists because one did on a similar job. Quoting a weldment without losing the fit-up works through where the hours actually go on a fabricated frame, and fit-up is usually the largest of them.