I have a single line diagram which mentions the transformer voltage as follows:

600/120/208V Could any one explain what does it mean?

Most likely it is 600 volt delta on the primary and 208Y/120V on the secondary.

600V?? where are you / what type of system?

Yeah I am there.. I got the answer from some other resource.

Details are: 600V (Primary Delta phase to phase) / 208(secondary phase to phase) / 120 (secondary phase to neutral).

It's a fairly common voltage in commercial settings in Western Canada..

I'd say it's really common across all of Canada.
Up here, 480 V is for foreign equipment

600V is the traditional transformer voltage rating on a system employing 575V motors, just as 480V is the standard transformer voltage on a 460V motor system. The difference in voltage ratings account for the voltage drop which almost always (except in the case of very short conductor lengths) exists from point of transformation to point of utilization. Very common in Canada and amongst process users (refiners, pulp producers, chemical plants and the like) in the US.

As stated, rating implies 3 Ph Transformer 600V primary, 208Y/120V secondary.

Foreign equipment?? Don't know that I'd use that term... To find 600 volt in the oil and gas industry in Alberta (using it as an example as that is where I spend most of my time) would be rare... There are thousands of 480 volt installations province wide, both 3 phase and single phase..

I agree there are some whole plants running on 480V in Canada, but it's not the standard across all sectors. In manufacturing plants (where I spend most of my time), 600V is standard, with the odd 480V transformer for one or two machines. Of course, even then there are some exceptions, where plants using a lot of foreign equipments add a 480V distribution across the whole plant.

Here (Province of Quebec), the local utility only provides 600/347V or 240/120V for low voltage. If you want 480/277V, you must transform it yourself.

This single line was from a building in Alberta, Canada.