The spid column of systransactions displays the Process ID of the process attached to the transaction (or 0 if the transaction is detached from its thread of control).
For local transactions, the spid value indicates a Process ID running on the local server. For remote transactions, the spid indicates the Process ID of a task running on the indicated remote server. The output above shows a spid value of 8 running on the remote server, caserv2.