3GPP 38.300 v19.3.0 — the document's own text
7.5 UE Capability Retrieval framework
Taught in 14. Conformance testing, and why a separate group writes the exam (TSG RAN — the group that specifies the radio, in depth), 2. The six working groups and the line between them (TSG RAN — the group that specifies the radio, overview).
The UE reports its UE radio access capabilities which are static at least when the network requests. The gNB can request what capabilities for the UE to report based on band information. The UE capability can be represented by a capability ID, which may be exchanged in NAS signalling over the air and in network signalling instead of the UE capability structure.
In IAB, it is optional for an IAB-MT to support UE capability Retrieval framework and the related signalling. In case IAB-MT does not support UE capability Retrieval framework, IAB-MT capabilities are assumed to be known to the network by other means, e.g. OAM.