you have very precise chas' answer, and as you were mentioning definition of the space (occupied or not), i assume that's what you want to hear.
if legislation does not concern you (at least us legislation), than i would recommend that you take a look at generator manufacturer's documentation.
if none of it is binding you, you could possibly omit ventilation in such a space, as you neither have direct pollution, nor occupancy demands, not you need ventilation for heat rejection.
chas, do you have some one-page table of new ashrae 62.1 demands to link at? i know that 62.1 is much more than simple table, but i would like to compare it with older version of standard that i possess.