Here (noris.de), a principal engineer is expected to:

* do some publications and/or publications in their field of expertise

* provide technical leadership (input on technology decisions, some architecture support, help write guidelines etc.)

* be part of the escalation path if problems cannot be solved otherwise

* help others in the company level up with their skills (at least as far as they are interested)

Here, principals are actively discouraged from becoming team leads (tech and people leadership are separate paths, which suits me quite well).