3GPP 21.900 v20.0.0 — the document's own text
4.10.3.3 Release naming
Taught in 3. What a Release is, and what freezing one costs (How standards actually get made, in depth), 4. The change request and the release (How standards actually get made, overview), 3. One idea, from proposal to published text (How standards actually get made, quick start).
GSM phase 2+ specifications were grouped into annual Releases from 1996 to 1999. The first 3rd generation specifications were grouped into an initial Release 1999.
Subsequent Releases are not necessarily annual, and shall be referred to as Release 4, Release 5, etc., according to the major field of the version number (see table 4 and clause 4.3).