The waste pipe on my bathroom basin has an extra pipe about 1/2" dia joining it just below the drain hole in the basin and before the trap. The pipe is flexible and goes off into the wall. Is it an extra drain for something? There is an air conditioning unit in the roof above.
The main waste pipe appears to be manufactured for this extra pipe so when I replace it what should I ask for.

Thanks for any help.

This is a vent to prevent airlock when the lavatory drains. This type of vent's added when the lavatory has no overflow installed. This has nothing to do with the AC condensate line.
" The main waste pipe appears to be manufactured for this extra pipe so when I replace it what should I ask for."
Just tell them that you have a bowl type lavatory with no overflow. Ask for a vented drain and tailpiece. Good luck. Tom