In the BIMrx MEP Settings dialog, the last section is Elbows(Conduit Parameters):

The Conduit Parameters works to automatically apply the specified instance parameters from the selected conduit to the conduits being created using the BIMrx Elbow commands.

In order to specify which parameters to be copied from the selected elements to the new elements using the BIMrx Elbow Commands follow these steps:

1. select the paramter(s) in the column on the left labeled Common Instance Parameters.

2. use the arrows in the center to move the parameters to the column on the left labeled Instance Parameters to be Copied. You can use the arrows to move paramters back and forth between the columns. 

3. Select OK when finished 

Note: You can also double click the parameters to move them from one column to the other