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It varies according to train type, company and refurb state. D is the carriage, 60 is the seat number, and A denotes 'airline' - i.e. Your seat faces another seat's back, instead of being one round a table.

There are some maps out there - but you'll need to say which service you're on (company and time/route) to pin it down...
By the way, seat reservations are rarely compulsory (only with megatrain and mobitix fares, technically) so if the train isn't too busy, you can sit where you want - every seat will have something telling you if it's reserved.