FWIW, the "principal" part implies a few things no matter which org... they should be able to manage a project from start to finish, mentor juniors, and make confident correct architectural decisions.

If you've ever hired a plumber or electrician, you might have gotten a crew of younger apprentices, maybe a journeyman, and an older "master" plumber or electrician. Most of the master's time is spent with the critical mechanical tasks, solving problems that occur, and directing those other tradesmen. The principal is the 1 person who can do _any_ of the other's job if they are unavailable. They are also the 1 person (and ideally the only 1 person) who makes "the plan" for how the work will proceed, and also decides when a project is complete.

The crucial difference (well, one of many?) between a principal-level engineer, and any type of management...is that the principal-level engineer should be able to do every junior engineer's job in a pinch - expertly, and with confidence and adapability to problems.