A RebarConstraintsManager is created by calling Rebar.GetRebarConstraintsManager(), and can only be used to query or change constraints on the rebar element that created it.

By default, Revit uses the following logic to choose constraints for each handle on a rebar element. First, a search is performed to find all suitable target planes, including surfaces of the rebar's host, as well as surfaces on other concrete host elements that are attached to the rebar's host. In the case of standard style rebar, any host surface occupied by a stirrup will be ignored, and instead, the handles on the stirrup itself will be treated as candidates to form a constraint.

Once all the constraint target candidates have been determined, the following sequence is used to select a constraint target:

1. If the rebar is a straight standard bar, it will search the list of candidates for stirrup bends to lock its RebarPlane and Edge handles. If a bend is found within tolerance distance, then the bar will be snapped and constrained to that bend.
2. The bar will snap and constrain its handle to any host cover or stirrup handle that lies within tolerance distance.
3. If no candidate is found within tolerance, then the bar will choose nearest host surface target, with or without cover, and create a constant distance constraint to that surface.

Snapping tolerances are 0.5 * bar diameter for host surface cover constraints and 0.5 * (bar diameter + stirrup bar diameter) for stirrup handle constraints.

The RebarConstraintsManager allows the API developer to obtain the constraint candidates for each constrained handle on a rebar, and to override the default target selection logic by setting a particular constraint as preferred. This can be useful in a number of ways. First, it can be used to snap a handle to a particular host surface or stirrup rebar handle, or to position a handle at a precise distance from a host surface. Second, it can force a rebar handle to constrain itself to a particular target surface, even if other targets are closer (or will become closer in subsequent updates of the Revit model). For example, a bar can be constrained to maintain a constant offset distance from a face of an opening in a slab, even if the opening is placed close to the edge of the slab and the bar would normally constrain itself to the slab edge. Lastly, the override can be used to cancel the default standard bar preference for stirrup bar handle planes, and to allow standard bars to be constrained to host cover surfaces, even when a stirrup is present.