STP - Topology Change Acknowledgement (TCA)
In an STP deployment, if there is a change in the STP topology, a series of BPDUs are exchanged between switches. Specifically, topology change notifications (TCNs) are sent upstream to the root bridge. In the process, topology change acknowledgments (TCAs) are sent in response to those TCNs.
A TCA BPDU is defined by a TCA bit set to 1. This bit is used as an acknowledgment response to a TCN BPDU. When a switch sends a TCN BPDU towards the root bridge to indicate a topology change, the upstream switch (and eventually the root bridge) will acknowledge this notification by setting the TCA bit.
The TCA bit is set by the upstream device (including the root bridge) in the configuration BPDUs sent back to the switch that originated the TCN. This bit essentially tells the downstream device that its topology change notification has been received and processed.