Multiple Boxes DCS User Model for tool

It gave you the answer. You can only use 1 box. Everything else must be a different shape type.

I would recommend not to use boxes for your user model, they take up a huge amount of your process time factor. This can be viewed at the top of the Cartesian position check page. Multiple line segments surrounding your tool are much more efficient.

I think a box takes up over half of the allowed time, which would explain why you can't use more than one.