Could someone please explain this Link type? Why/how would you use it as opposed to a One to Many or Many to Many.
If you have a couple of forms that already have a common field, then the Join Link Type is great for establishing a relationship between your forms. For example, if you have an Order form and an Order Items form and they both have an Order Number field, then you can join those two forms together via the common Order Number field.
The other Link Types are for manually establishing relationships between forms. The Join type just looks for a common value in the parent field and the child field and links the records together.
Hope that makes sense.