Serial Port Monitor is divided into Professional and Company Editions.

The difference you can see here:

Number of computers per license110
Any serial port software/hardware can be analyzedyesyes
Monitor any number of serial ports at onceyesyes
Supports all COM-port typesyesyes
Intercepts all data read from/written to COM portyesyes
Monitoring serial ports which are already in useyesyes
Sending/receiving data to/from any COM port (Advanced Send Dialog)yesyes
The possibility to change COM port parameters from Advanced Send Dialogyesyes
1 license can be installed on 10 computersyes