3GPP 38.300 v19.3.0 — the document's own text

5.3.5.3 Uplink timing control

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).

The gNB (including IAB-DU and IAB-donor-DU) determines the desired Timing Advance setting and provides that to the UE (or IAB-MT). The UE/IAB-MT uses the provided TA to determine its uplink transmit timing relative to the UE's/IAB-MTs observed downlink receive timing.

An IAB-node may support additional modes for uplink timing:

The IAB-MT uses the provided TA plus a provided additional offset to determine its uplink transmission timing, to facilitate parent node's IAB-MT Rx / IAB-DU Rx multiplexing;
The IAB-MT aligns its uplink transmission timing to that of the collocated IAB-DU downlink transmission timing, to facilitate IAB-MT Tx / IAB-DU Tx multiplexing of this IAB-node.

The IAB-node uplink timing mode is indicated by the parent node via MAC-CE.