At this time, that decision is mostly being left up to the community. The general content submitted so far seems to be music, film, and television related. At this time, the size of the posters submitted range from 9" x 9" album promotionals to oversized posters.
A credit is for the artist, company, or creator(s) involved in producing the poster itself. This can be a graphic designer, illustrator, printing company, design company, etc. If they are credited in the production, they can be listed in the Credit field.
A subject is anything of primary focus for the poster. It could be a musical artist, an album being promoted, an event venue, a movie or TV show, or a musical, film, or cultural festival. Basically, if it is listed on the poster, then it is a potential subject. Do your best to fill out details on the subject including photos. When possible, insert the link to musical artist pages at Discogs and movie pages at Filmogs. This type of crosslinking will help bring exposure to the other sites as well.