Surface finish should connect to function: sealing faces, bearing fits, cosmetic areas, and bonding surfaces often need different controls. A common approach is to call out Ra (or Rz) on critical faces and use a general note for “as-machined” elsewhere.
For cosmetic parts, specify whether tool marks are acceptable and whether directional grain (brush) is required. For coatings, note masking boundaries and any blast or prep steps.
If only one side of a plate must be flat for gasket sealing, say so explicitly—otherwise shops may assume uniform finish across non-critical faces.