Naming convention for procedures, from the ICAO bible...
9.5.3 Duplicate procedure identification
9.5.3.1 A single letter suffix, starting with the letter z following the radio navigation aid type shall be used if two or more procedures to the same runway cannot be distinguished by the radio navigation aid type only. For example:
VOR y Rwy 20
VOR z Rwy 20
9.5.3.2 The single letter suffix shall be used when:
a) two or more navigation aids of the same type are used to support different approaches to the same runway;
b) two or more missed approaches are associated with a common approach, each approach being identified by a single letter suffix;
c) different approach procedures using the same radio navigation type are provided for different aircraft categories;
d) two or more arrivals are used to a common approach and are published on different charts, each approach being identified by a single letter suffix. If additional radio navigation aids are required for the arrival they shall be specified on the chart's plan view. For example:
ILS y RWY 20 (“CAB VOR Arrival” shown in the plan view)
ILS z RWY 20 (“DNA VOR Arrival” shown in the plan view)
9.5.3.3 As some avionics systems are capable of loading only a single approach, States should ensure that the preferred approach is identified using the z suffix.