The RelationshipManager system is for making MOOSE aware of extra Geometric or Algebraic information needed to perform a calculation. This is an extension of the "stencil" concept when working with a structured mesh and performing calculations where you need neighboring information. Relationship managers (RMs) come in two flavors: Geometric and Algebraic. Geometric RMs are useful for making sure extra mesh elements beyond your individual processor partitions are available. In DistributedMesh mode, you can only count on having one extra layer of neighboring elements available outside of your partition.
The RM system is fully pluggable like other MOOSE systems but it not intended to be exposed in the input file in any way. Instead, individual objects that require extra information should "register" an appropriate RM right in its
validParams function and have MOOSE add the appropriate object to the simulation at the proper time. It is possible for an RM to supply by Geometric and Algebraic information.
Geometric RMs are not necessary (active) when running MOOSE in ReplicatedMesh mode.
Geometric Relationship Managers
Geometric Relationship Managers are useful if a user requires extra "halo" information around a partition. Use cases may include Reconstructed Discontinous Galerkin (rDG) or the use of the GrainTracker in the PhaseField module.
Algebraic Relationship Managers
Algebraic Relationship Magers are useful if an objects requires extra solution or field information from elements beyond the current processor's partition.