If by flat you mean that we don't have a bunch of bureaucratic hierarchy, that's true. We have Engineer 1, 2, and Senior on the individual contributor line, and Lead, Manager, Sr. Manager, Director, and Sr. Director on the personnel management line. It could probably be simplified further without losing scope of responsibility information. Right now we have about 50 individual contributors and 11 personnel managers. Keeping a ratio of 5 individual contributors or less per manager makes it possible to have real career development conversations regularly.

Senior Engineer carries high expectations around scope, impact, and autonomy. In other words, Seniors have a lot of responsibility, are expected to move mountains regularly, and operate with relatively little direction.