The multiple values for each voltage range refer to the electrical condition of the surface behind the worker when they are facing the energized parts of the equipment. Condition 1: an insulated surface is rarely encountered in practice; Condition 2: a grounded surface; and Condition 3: other electrical equipment that may be energized are the most common. Condition 3 must be applied for aisles with electrical equipment on either side even if it is not likely that equipment on opposite sides will be opened for service at the same time.
If the equipment design does not provide full access to all electrical components and connections from the front of the enclosure, then this clearance is also required behind or beside the gear where such access is provided. It should also be noted that these NEC clearance requirements are adopted by OSHA in their work regulations. While some electrical inspectors may accept lesser distances in existing installations that are "grandfathered," under OSHA, electricians cannot work on such installations when they are energized.