3GPP 21.900 v20.0.0 — the document's own text
6.4.2 Choice of deliverables
Taught in 6. Work Items and Study Items — how the work is authorised (How standards actually get made, in depth).
The SI will be realized as a new Technical Report ("feasibility study report").
The WI will be realized as new specifications and/or amendments to existing specifications; the exact structure lies with the individual TSG Sub-Groups and the TSG. Typically, a new feature may result in at least three completely new specifications (stages 1, 2 and 3) but may also cause amendments to the major protocol specifications.