If you have a u-bend you have two axes in it, say AxisA and AxisB. These two axes have to be constrained to another two axes which represent the tubes which the u-bend connects, say Axis1 and Axis2:
The problem is that although you can fix AxisA to be completely constrainged by Axis1, depending on the accuracy of the parts you may not be able to constraing AxisB to Axis2. For example the inter-axis distance might be 25.0 on the u-bend but 25.06 between the two tubes.
You can get over this problem by changing the parameter in the second constraint. If you set it to 0.1mm, as shown above, it means that one of the axes rotates around the other with a radius of 0.1mm:
In the first image above the Mate:12 (0,100) tells you the "radius" "offset" of 0.1mm.
If you don't give this flexibility in a 4 axis construction you might get a yellow rectangle liks this:
