Size of measurement unit ?

Hi this may be a daft question
What is the arbitrary size of a unit in AI ? just experimenting on working out projector positioning in the 3d space and calculating distances

Many thanks
Chris

Just had another look at the manual and have seen the nominal value in metres missed that bit in previous reads
Cheers
Chris

It just uses 'generic units'. So providing you use the same units throughout your project then you should be fine. Some customers treat the units as inches or feet whilst other customers treat them as meters. Personally I prefer meters!