BOP Rams Types, Applications & Features
The BOP rams are the result of some eighty years of development in the BOP stack system. The first BOP ram was developed in the early 1900s. The ram preventer will only seal on the specific condition for which the ram block is designed.

Rams are interchangeable between ram-type preventers of the same design and pressure rating. Casing rams are usually substituted for pipe rams before casing running or liner running operations.

Figure 1 : Ram Preventer (Hydril)

Most
https://www.drillingmanual.com/bop-rams-types-applications-features/