Opening (OP) Cel, Ending Cel (ED)
These are the cels that make up the opening of the show. Most shows have 1 opening that is animated per season and then the sequence is reused for every episode. The same applies to End credits. Only one set of cels is made for each opening or ending. These cels are generally the most sought after as most fans of the show find them easily recognizable. They also tend to be super pricey as well so they are often a target for for fakes and reproductions being sold as the real thing, so watch out!
This is an example of the SDFM Macross TV series opening credits. Artwork from this sequence is very rare and expensive.