My understanding of these agreements is that they are a better fit in Relevant Communications (despite generally trying to avoid that artifact) because the nature of an agreement has some kind of scope whereas “Compliance Documentation” is for the IRB as a whole.
Depending on your perspective I could see it falling into either Zone 4 (if you are a sponsor and filing a reliance agreement for an IRB that will cover multiple sites) or Zone 5 (if you are a sponsor and filing a reliance agreement for a single site’s local IRB).