The Muscle Creator window lets you use naming conventions to mirror muscles after they have been sculpted. The muscle for each “side” needs to be parented to joints/nodes that follow a Left or Right naming convention (or something similar, like Front, Back), and each muscle must have been created with that naming style. For example, if your naming convention is to differentiate between left and right using _L and _R, the first muscle you create should be named something like Bicep_L or Bicep_R.
