The various dominios in Might & Fealty all interact with each other, and aside from the things they do to each other on the battlefield, there are also the formal relations between realms, which is what we call diplomacy.
The realm hierarchy (see dominios) should be considered a part of diplomacy as joining or leaving realms or changing your position in the realm hierarchy is very much a diplomatic act and can be used politically in many ways.
The unusual concept to digest is that in Might & Fealty, diplomacy has no game mechanics effect whatsoever, intentionally. Just like other parts of the game and just like real life, your diplomacy is only as strong as your ability to execute and enforce it. There will never be a red banner saying "you cannot attack this settlement" just because some diplomat far away signed a peace treaty. If the ruler is unable to prevent his vassals and realm members from continuing the war, then that peace treaty is not worth the paper it is written on.
Realm relations are defined by the ruler or anyone granted the diplomat status (see sistema de permisos). They are unilateral, so you can declare someone as a friend who thinks of you as an enemy if you find some sane reason to do so. They consist of the following parts: