you can use that as restraint (constraint? I can never remember the difference).
See here: https://www.phenix-online.org/version_docs/1.9-1692. Short version: restraints keep parameters similar (or moving together), effectively adding "observations"; constraints actually reduce the number of parameters. For example, the individual_sites and individual_adp strategies in phenix.refine use restraints (on geometry and ADP similarity), while the rigid_body and group_adp strategies use constraints (treating groups of atoms collectively).
So yes, "restraint" is the correct term for the reference model feature. But it's amazing how often I see the terms confused in methods sections!