Changed Block Tracking (CBT) is a VMware feature that tracks changed disk sectors and improves the performance of VM backups. By default, the vSphere Agent enables Changed Block Tracking (CBT) for VMs.
However, because CBT requires some virtual disk processing overhead, you can stop the Agent from enabling CBT for VMs. This does not disable CBT for VMs that already have it enabled through the Agent or another mechanism. It only stops the Agent from enabling CBT in the future for VMs that do not already have it enabled.