I'd like to ask about the meaning of 'one' in the following context:
We know that one principal condition, at least for the overwhelming
majority of such images, appears to be the use of an
electronic flash. We also know that using a digital camera, as
compared to a conventional camera using film, is almost a requirement.
Does 'one' here mean "the only (one)" or does it mean "one of", i.e. there are several principal conditions and the use of an electronic flash is one of them?