Seligman's system is based on virtues, and subordinate to the virtues are personal strengths. He determined the virtues by having a team study philosophies from around the world and from various eras for commonality, and from that compiled a list of "universal" virtues. You fill out a questionnaire to find out which are your signature strengths from a list of 24 strengths. Your signature strengths are 3-5 that score highest on the questionnaire. So his method is a combination of objective and subjective, but starts with the objective and determines the subjective from there.
It's an interesting method, but for me it seemed useful more as a brainstorming tool, perhaps useful to get some personal insights just by means of a fresh point of view. It's too formulaic to be of much further use, at least for my taste.