In the branch table, if you leave certain combinations of nominal diameters empty, you can still create such combinations by Plant 3d default connection behavior. But if you create “warning elements” as Tees and assign them to the unwanted combinations, the piper will clearly know those are unwanted and he will not be able to create those. You just have to create a new family of Tees, name it properly and change the design parameters to the following:
L1, L2, L3 to 0.01
D1, D2, D3 to 5x original value
Then in the branch table, add the new Tees, name them “ERR” and assign them to the unwanted combinations of nominal diameters.
As a result the piper will see the following, if he tries to create an unwanted combination:
(0)