Whats Relationship in BPF?

Whats Relationship in Business Process Flow?
So when you add stage, and select some different Entity, Some time you get option of Relationship and sometime you don’t see that area Why?

This behavior of BPF is because of relation between entities selected for stage.
If first stage is account and next stage is contact and there is I:N relationship From account > contact then you will get an option to select relationship, else you will not get any option.

Now what this relationship option will do if you select any relationship and what will happen if none is selected?
So first thing, it will list all the 1:N relationship you have with next stage.
When Relationship is selected as none and you click Next stage.
You see one fly-out next to next stage button which will say no contact, create new.

Now when we select some relationship and update BPF and again click next stage, you will again get fly-out but this time it show all the related records of contacts based on relationship provided.
For E.G if there already 5 contact record where company Name(account) is current account, Then all those 5 records will be available,
This is for reducing duplicate record creations.