After you get your queue number, the first counter is L27 (sometimes L28), it has a sign " document check" and will flash your queue number when it's your turn.
The lady there will check whether you have all necessary documents, put them in the correct order, give you back some unnecessary photocopies and add some unnecessary photocopies she deems necessary.
She will also give you some (3-5) forms to sign (no real need to fill them out, just sign when you are waiting for the I.O. at L35), like the one where you state that you are a nice person or the one where you sign that overstay carries certain penalties.
She gives you back all your documents, you then wait at e.g. counter L35 for your queue number to flash, and there the I.O. stamps the extension stamp into your passport.
I don't know if your queue number will ever be called at L35 if you didn't go to L27 first.