3GPP 21.900 v20.0.0 — the document's own text
4.10.1.0 General
Taught in 3. What a Release is, and what freezing one costs (How standards actually get made, in depth).
The concept of Releases was introduced in clause 4.0B. A given specification may simultaneously exist in several versions, each corresponding to a different Release.
In principle, a Release of the specification can be identified as consisting of all those specifications with a "major" version field of a given value.