It's pretty straightforward, especially for the first 4 major levels:

- salary bands (+ stock awards & bonus. i.e "TC" bands)

- average seniority

- title change ( I (entry level) -> II -> Senior -> Principal -> ...)

- internal ladder descriptions (someone who worked at at least 2 FAANG companies can trivially figure out what maps to what, the titles & on-paper responsibilities are pretty standardized across most FAANG)

Hi, one of the founders of Levels.fyi here (appreciate the kind words)! The question we try to answer with our leveling comparisons is “What level would I transfer into if I were to switch companies?". To answer this, we collect company leveling rubrics from employees and crowdsourced transfer mappings, and use it to match levels based on similar scope and responsibility.

There are also a lot of factors on an individual case by case basis that go into leveling including: interview performance, tenure in level, and scope of work, which is why it won't always perfectly map 1:1 for all cases and hence why some levels are wider and overlap into more levels at another company. And beyond that, leveling is a living and breathing thing within any organization and may be in flux at any given point in time. At the end of the day, the site is meant to serve as a rough guide for where you may be placed. Also note that the leveling boxes actually have no bearing on compensation.

Hope this helps!