2, 3U panels in rear door can accept fan panels or vent panels. Rack ships with solid steel filler panels in rear door 3U openings. Rack has a steel frame under the wood outer covering for strength, stability, and to prevent caster tear-outs. Casters can be replaced with floor levelers (P/N FL) for permanent installations. Front and rear rack rail are included and are adjustable. Vented, removable bottom cover panel provides extra room for component installation, but can be easily removed for cable entry. Ships fully assembled.