When creating a subassembly using Subassembly Composer the hardest intellectual thing is knowing if I should use Output Parameters. It’s not hard in creating them, it’s hard in knowing if another subassembly that you have or haven’t thought of yet will need a value from your subassembly. An output parameter essentially lets you provide information of your subassembly to another subassembly other than the point of attachment. For instance you could provide a slope of the roadway pavement as an output parameter. This might be handy if you wanted to match slopes with a subassembly that does not support superelevation.
To use the output parameters in another subassembly you have to set them from the subassembly properties dialog. Select the parameter you want controlled and then you should be able to select an output parameter of a subassembly within the currently attached Assembly.