Rack shelves and accessories are equipment used to mount equipment in racks. All the equipment is based on the TIA standard 19” rack mount specification. Most rack mount equipment is based on 1-3/4” height increments. 1-3/4” is considered 1 RU or a Rack Unit.
Rack shelves are for mounting “non-rack mount” equipment, blank panels are for spacing between equipment, rack screws are for mounting equipment and fans are for thermal control. There are literally hundreds of accessories available for racks.