The rule is this:
First, branches match to branches
Then, items match to items.
Both branches and items use basically the same “matching” rules - effectively, “longest list” matching. So if you match 1 branch M with many branches N1-N6, it will match M to N1, M to N2, etc. If you have M1-M3 branches in one input, and N1-N3 branches in the second input, it will match M1 to N1, M2 to N2, and so on.
Then, once the branch associations have been made, it handles item matching within each branch match, in exactly the same way.
So items match like this:
and trees match like this:
So this means when you graft both grp 1 and grp 2, it’s just going to match each branch with the corresponding branch in the other list. Each branch has only one item, so after that, it’s done!
Hope that helps explain a little bit; if theres a particular problem you’re trying to solve, try to explain it a little deeper and we can try to help 