Click on Self-Collisions on the left pane and select Generate Collision Matrix. Here, you can set the sampling density high if you wish to move the robot arm in a more confined space. This may increase the planning time for the robot to execute a trajectory and may sometimes fail execution due to a collision assumption. However, we can avoid this failure by manually defining and checking the collisions for each and every link and disabling or enabling them accordingly. Let's not assume we have a virtual joint for our robot.