Dell Precision 7920 Rack Owner's Manual page 70

The static rails support a wider variety of racks than the sliding rails. However, they do not support serviceability in the rack and are thus
not compatible with the CMA.
One key factor in selecting the proper rails is identifying the type of rack in which they will be installed. Both the sliding rails and the static
rails support tool-less mounting in 19"wide, EIA-310-E-compliant square hole and unthreaded round hole 4-post racks. Both also support
tooled mounting in threaded hole 4-post racks, but only the static rails, as the more universal solution, support mounting in 2-post (Telco)
racks.
The table below shows the sliding and static rail configurations and supported racks:
70 Removing and installing components