Crests are universal upgrades which can be used at any level 80 item with an upgrade slot.
Relation to other upgrade components
Crests belong together with a set of upgrade components which provide flat bonuses to various attributes.
Seals, Medallions and Crests have the same set of suffixes, the Crest is the stronger version of the Medallion, which is the stronger version of the Seal.
Marks and Talismans are the predecessors of the Seal, but have an entirely different set of suffixes.