Domain depth
Sustained, multi-year work in one technical area. It is the clearest form of demonstrated expertise, because it accumulates only through time.
What it tells you about a developer
Strong domain depth in the exact area your role needs answers much of the hiring question up front. This person has already spent years on the exact class of problem you want solved.
Depth accumulates through time and repetition in one territory. Domain depth is a developer returning to the same problem space across projects and years, with the work getting harder or more ambitious each time, rather than staying at the same level.