In the initial iteration of simultaneous multi-robot cells, there is limited external axis support.
The cell editor interface prohibits any unsupported external axis use cases, so there is no risk of creating unusable cells.
Limitations:
Parts cannot be mounted on external axes. They must be mounted on stationary objects. This means that path mounting frames cannot be children of external axes in the cell tree.
Robots cannot be mounted on rotary axes.
Robots can be mounted on rails (including gantry systems).