What is an as-built?
An as-built is the record of what was actually installed, as opposed to what the plan called for: real depths, real offsets, real materials, captured before the trench is backfilled.
Plans say what should happen. Ground conditions, utility conflicts, and field decisions change things, and the as-built is the record of what really happened. For dirt and utility work that usually means pipe depth and location, fittings and materials used, where lines cross, and distances from permanent reference points like foundations or curbs.
Why it matters
Whoever digs there next, including you in five years, needs to know what's down there. Municipalities and utilities often require as-builts for acceptance. And when a customer claims the line was put in the wrong place, the as-built is the evidence that it wasn't.
The practical way to capture one
Photograph the open trench with a tape or grade rod visible, then write depths and offsets right on the photo before backfill. A photo with markup, a date, and a GPS location beats a sketch from memory at the end of the day, and it takes thirty seconds while the hole is still open.