The DF500 has a limit on how deep can drill the mortise, so you have to improvise to use it to maintain the structural integrity with an small tenon.
The use of multiple dominos comes to mind to fix that problem.
The DF700 cannot use small bit, it starts from 8mm. So you cannot use it with plywood 1/2 or 3/4
If you want to use a small bit 4, 5 or 6 mm with the DF700 or you want to have a deeper mortise that the DF500 then the Seneca adapter comes handy. You can use the 4,5,6 mm with the DF700 and also have a deeper mortise. That is why I call it a 3 domino (DF600 comes to my mind

)
It bridges between the difference of the 2 Dominos. I hope I make sense.