3GPP 21.900 v20.0.0 — the document's own text
4.10.3.5 Early implementation of features
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).
3GPP may identify certain features as being suitable for "early implementation". The selection of "early implementation" features shall be performed at the TSG level.
Additional documentation is provided for early implementation features (see clause 6.4.4).
All documentation for early implementation features is contained in the Release where the feature is introduced. The status or specification of older Releases shall not be changed by the introduction of early implementation features.