Principal Software Engineer
A Principal Software Engineer is a very high ranking of software engineer, above senior staff software engineer, staff software engineer, senior software engineer, etc, but below distinguished software engineer.
Sometimes an engineer is a principal software engineer because the company cannot do without them because that person is the only one who completely known their software system.
But more often, as principal software engineer is someone who has very extensive industry accomplishments, can do all the work on a project completely own their own and independently, and has the ability to be engineering lead for a small team (of say 3 to 7 engineers)
Typically, principal software engineers are self-motivated to the point that they create new work and have their own vision for what needs to be done.