A 23-inch rack is used for housing telephone (primarily), computer, audio and other equipment though is less common than the 19-inch rack. The size notes the width of the faceplate for the installed equipment. The rack unit is a measure of vertical spacing and is common to both the 19 and 23-inch (580 mm) racks.
Hole spacing is either on 1-inch (25 mm) centres (Western Electric standard), or the same as for 19-inch (480 mm) racks (0.625 inches / 15.9 millimetres spacing).
Another name for this type of racks is ETSI rack, relating to the European Telecoms Standards Institute.