Skill level is not a ranking and it is not a judgement. It exists for one reason: so the games are close. A close game is a good evening; a one-sided game is an hour wasted for both people.
Here is what each level actually looks like, so you can place yourself honestly.
Beginner
You can rally, and you drop the rally more often than you would like. Your serve goes in most of the time. You know the scoring. You are still thinking about where to stand.
The defining feature is that the rally ends because of a mistake rather than because of a winning shot. That is not a criticism — it is true of most people who play, and beginner sessions are the most sociable ones at any venue.
Somebody who played at school ten years ago and has not played since is a beginner. Fitness fades, but technique fades faster.
Intermediate
You can keep a rally going reliably and you end points on purpose. You have a smash that lands in. You move back to the middle after a shot without thinking about it. You can play a drop that is not obviously a drop.
In doubles you understand front-and-back and side-by-side and you switch between them without discussing it.
Most club players are here, and many stay here happily for years.
Advanced
You have deception. You can hit the same preparation and produce three different shots. Your footwork is efficient rather than fast — you arrive early rather than lunging late. You control the pace of the rally rather than reacting to it.
You are also, almost certainly, someone who has had coaching.
Why people place themselves wrongly
Upwards, out of pride, and then spend the hour picking the shuttle off the floor while three other people wait.
Downwards, out of politeness, and then dominate a game nobody else enjoys.
Both are worse than an honest guess. If you are unsure, say the lower one and tell the coach — moving somebody up after ten minutes is easy, and moving them down mid-session is awkward for everyone.
Levels change
A coach sets and adjusts your level as you improve. That is the point of having a coach watch rather than a form to fill in: progress in a racquet sport is uneven, and it shows up in specific shots before it shows up in results.
At KGB, groups are set by skill rather than by age, so a fourteen-year-old and a forty-year-old at the same level train together. It works better than it sounds.