The Side Clearance value is the length of a linear move that is perpendicular to the first move of a toolpath.
The most common use for this parameter is to establish a G01 move during which the control can apply the cutter the compensation offset (D-Comp) prior to beginning the actual cutting motion. When Cutter compensation is being used, 10% of the tool radius is a good starting value. You can adjust this value for specific situations.
Another common use for this parameter is to keep the tool a user-defined distance away from the finish contour for the plunge move to the Z-Level of the tool path. Typically this is used in pocketing with islands when no Leadin move or Leadout move is programmed. This keeps the tool from leaving rub or chatter marks along the wall of the island or profile of the pocket during Z plunge and retract moves.