L3 is new grad. L4 is "terminal" in that there's no explicit up-or-out pressure beyond that point. L5 is senior, and I'd say most people don't make it beyond L5 without intent. L6 has a few shifts from prior levels. Notably that a high performing L3 is an L4 and a high performing L4 is an L5 (usually), but simply performing well at L5 isn't enough to get to L6, the scope of work and goals shift between L5/6. You generally won't be considered for L5 without 10+ yoe, as I understand it, although internally you can reach it faster.