3GPP 38.300 v19.3.0 — the document's own text
4.5 Multi-Radio Dual Connectivity
Taught in 4. What NR is — the shape of the 5G radio network (TSG RAN — the group that specifies the radio, in depth), 4. What NR actually is, in the shape its own document gives it (TSG RAN — the group that specifies the radio, overview), 3. What NR is, read off the document itself (TSG RAN — the group that specifies the radio, quick start).
NG-RAN supports Multi-Radio Dual Connectivity (MR-DC) operation whereby a UE in RRC_CONNECTED is configured to utilise radio resources provided by two distinct schedulers, located in two different NG-RAN nodes connected via a non-ideal backhaul, one providing NR access and the other one providing either E-UTRA or NR access. Further details of MR-DC operation, including Conditional PSCell Addition (CPA) and Conditional PSCell Change (CPC), can be found in TS 37.340 [21].