3GPP 38.300 v19.3.0 — the document's own text
5.4.3 Uplink Tx switching
Taught in 2. The six working groups and the line between them (TSG RAN — the group that specifies the radio, overview), 2. The six groups, and what each one worries about (TSG RAN — the group that specifies the radio, quick start).
In uplink CA or SUL, a UE configured with uplink Tx switching can have Tx chain(s) dynamically switched from one uplink band or two uplink bands to another uplink band or two uplink bands for enabling up to 2Tx UL transmission in one uplink band or simultaneous UL transmissions in two uplink bands at a time.