This value controls which direction of traffic on a source interface gets mirrored to the group's destination port: none(0) mirrors nothing, onlyTx(10) mirrors only transmitted frames, onlyRx(20) mirrors only received frames, and bothTxRx(30) mirrors both, per the source's enumerated values. A network administrator sets this depending on what they're troubleshooting, for example mirroring only received traffic to check what's arriving on a suspect uplink without doubling capture volume with outbound traffic too. For example, setting tnMirroringIfMode to onlyRx(20) on a port under investigation captures just the inbound frames for analysis.