From the manual...
Cheers in advance."When inserting a VST plug-in into a rack, you don't immediately see the extra inputs/outputs of a VST plug-in. If you want to have control over these auxiliary inputs/outputs there are 3 options:
Insert a MUX in that rack and then insert the VST in that MUX. Then in that MUX's modular area you'll see all the inputs/outputs of the VST and you have all creative freedom to use them as you want. Also note that another advantage of working this way is that you can save this MUX as a preset that can be re-used in other projects, for example you can make VST presets that include a complete sub-mix setup!
Right-click the VST plug-in in the rack slot and choose "Setup Aux Ins/Outs". Then you can choose which auxiliary inputs/outputs should be imported/exported from the rack. When that's done the rack will have the extra inputs/outputs that directly represent the auxiliary VST inputs/outputs. Then go to the modular area containing that rack and there you can connect things as you want. If you want certain VST plug-ins to always import/export certain auxiliary inputs/outputs whenever it's plugged in, you can define this in the VST Plug-In Manager -> right-click the VST plug-in -> Edit Extra Options. This avoids having to do "Setup Aux Ins/Outs" again and again if you want that to be default behavior.
Insert the VST in the project's modular area and connect things as you want, similar to the first option, but here you can also add racks to the VST outputs and so these racks will also show up in the rack mixing desk.
