In 3.5.x and earlier versions, the null relation is used. In 3.6,
default was introduced, and null was deprecated.
TODO: "any" relation
TODO: high-order roles
There is also special pseudo-relation
any. This pseudo-relation cannot be used in object references. But it can be used in search filters for searches that look for references of all relations.
Since midPoint 3.9 new relations can be configured. Those new relations should behave in the same way as pre-configured relations.
Please see Relation Configuration page for more details.