L3 is early career, L4 is mid-career, L5 is senior. You can hit L5 on the strength of pure technical contributions regardless of business/org needs, usually.
L6+ is staff, and tends to involve a very different skillset. (If you're not looking to lead a team, you're probably not going to have the kind of impact that gets you to L6, let alone L7 or higher at Google.)
This is all to say that ICs in the L3/L4/L5 bucket generally show a clear progression in technical skills but beyond that it's fuzzier.