Parse Platform on iOS: Relations, Joins, or Arrays for Large Many-to-Many?

In the Parse.com API reference for Swift on iOS, it is very clear when to use the different kinds of One-to-Many relationships, based on the expected size of the Many side.

But I find it less clear on what kind of Many-to-Many relationships to use when both sides could be very large.

In my case, I have a Charity object that my Users can make small (often one-dollar) contributions to--so each User could conceivably make thousands of these contributions, and each Charity could have thousands of Users making contributions to it.

The Many-to-Many options listed for this kind of thing are Parse Relations, Join Tables, and Arrays, of which the docs explain:

• Arrays should be used when the relationship will reliably include under 100 references, which is very clear and helpful guidance that I should not use Arrays.
• The docs say Parse Relations could be used, for instance, to connect Books with multiple Authors and Authors with multiple Books--a situation in which a given Book is unlikely to have over 100 Authors, and only rarely will an Author have over 100 Books--so it's unclear if this is appropriate when both sides could be very large, as in my case.
• The docs say Join Tables should be used when extra metadata should be attached to each relationship, so for one thing, I don't at present have an explicit need for this, and for another, the docs don't seem to even mention anything about how or if it matters how large each side of the Many-to-Many relationship is.

In the absence of any other information, it looks like I should use Join Tables, but only because the docs don't imply that I shouldn't, and not for the reason the docs say I should.

Which seems like a flimsy rationale.

I would greatly appreciate any guidance anyone can give.

Behind the scenes, when you use Relation, Parse Server automatically creates a Joint Table for you and delivers some APIs for easily managing and fetching its data. So, in terms of performance, it should be very similar.

The downside of the Relation is the impossibility to add new fields to this "Joint Table" it creates. So, if you need, for example, to store the charities that each of the users like, a relation between User and Charity would be a good fit, because you just need to store that the relation exists and do not need to store any extra information.

On the other hand, if you need to store the donations that each user did to each of the charities, I'd create a Joint Table called Donation or UserCharity with a pointer to the User class, a pointer to the Charity class, and the value of the donation. In this case, Relation is not a fit because you need to store the donation value.