Answer copied below from ICAO Doc 8168:
8.5.1.3 Duplicate procedure identification
8.5.1.3.1 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 Z Rwy 20
VOR Y Rwy 20
8.5.1.3.2 The single letter suffix shall be used as follows:
a) when two or more navigation aids of the same type are used to support different approaches to the same runway;
b) when two or more missed approaches are associated with a common approach, each approach shall be identified by a single letter suffix;
c) if different approach procedures using the same radio navigation type are provided for different aircraft categories; and
d) if two or more arrivals are used to a common approach and are published on different charts, each approach shall be 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 Z RWY 20 (“DNA VOR Arrival” shown in the plan view)
ILS Y RWY 20 (“CAB VOR Arrival” shown in the plan view)
