Principal Software Engineers are essentially the Engineer Owner of a solution or closely related set of solutions.
They're expected to know the ins and outs of the solution both at a business level ('Hey, Principal, how much would it cost and how long would it take to do 'x'?' -- Some Sales Person), and have responsibility for the solution's engineering ('Two months? Great! See you at the February demo for the users group!' -- Some Sales Person), and be able to solve the more difficult engineering issues that crop up ('The app you promised to integrate with by February only uses SOAP and consists of around 14,000 on-prem installs...I graduated after the year 2000 so I'm not sure how SOAP works, can you walk me through this 'wizdal' thing?' -- Some Senior Engineer that you asked to look into the new project).