3GPP 38.300 v19.3.0 — the document's own text
4.7.3.3 Uplink Scheduling Latency
Taught in 4. What NR actually is, in the shape its own document gives it (TSG RAN — the group that specifies the radio, overview).
The IAB-node can reduce UL scheduling latency through signalling of a Pre-emptive BSR to its parent node. The IAB-node can send the Pre-emptive BSR based on UL grants it has provided to child nodes and/or UEs, or based on BSRs it has received from child nodes or UEs (Figure 4.7.3.3-1). The Pre-emptive BSR conveys the data expected rather than the data buffered.
a) regular BSR based on buffered data,b) Pre-emptive BSR based on UL grant,c) Pre-emptive BSR based on reception of regular BSR