Decision Computer International Watch dog 1 Operation Manual page 57

Operations Manual Watch dog timer card
Data Type
Long
Remarks
Port 0 is often used to establish connections between computers
dynamically. For example, a client that wishes to be "called back"
by a server can use port 0 to procure a new (random) port number,
which can then be given to the remote computer for this purpose.
57
DECISION Computer International