Cancel always uses a Command Sequence Number of 0. The cancel command uses the Process ID field to identify which SMB2/Notify to cancel. The PID must be the same as the PID of the initial STATUS_PENDING reply and the P bit must be set to 1 to indicate that a PID is present.
Cancel is used to abort any SMB2 commands that are "pending", i.e. that could not be completed immediately and where the server responded initially with STATUS_PENDING. Some commands that may not complete immediately and may need to be later aborted using the Cancel call are SMB2/Notify calls but calls to read from a named pipe such as SMB2/Read and SMB2/Ioctl can also be "pending".
When a call can not be completed immediately and the server responds with STATUS_PENDING the server will specify TID=0 in the SMB2 header and it will also specify a ProcessID (PID) and set the P bit to 1 in the response. Later when call actually completes the server will respond with the same command sequence number, TID=0 and the same ProcessID as the P bit set to 1. If the client aborts the call prematurely by calling Cancel the client will use command sequnce number=0 and the same ProcessID value as in the "pending" response and the P bit set to 1.
SMB2/Cancel Request Packet Format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Buffer Code | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There is no response packet.