An "affiliation tree" is still under development that determines which entities can participate in attacks against other entities within its hierarchy.
There is guild affiliation, there's party affiliation, there's a raid affiliation, alliance affiliation, there's a citizenship affiliation, there's society affiliation, there's religious affiliation. All of these things have some hierarchy; and within that hierarchy there's the ability to participate within certain systems. So for example, if you have a node that has fallen under your vassal state and you're a citizen of the parent node, then you could participate in a siege against the vassal node but if you're a citizen of the vassal node you could not participate as an attacker against the parent node; so there's a hierarchy, unless you were to renounce your citizenship. – Steven Sharif