Party lines(PLs) are used to tie a group of people together without assigning them to a Group. This gives the user the control of whether or not they hear/talk to the "conference". PLs are typically assigned as a "Split Talk/Listen" assignment on the VCP(VCOM Control Panel) of a user to give the ability to turn the listen off/on.
Here are a couple of tips:
When defining party lines, name them according to function.
Typical Production Crew PL setup:
This way each crew of a production can talk all together instead of having to open talks to each individual user. The Production Manager would have the capability of talking/monitoring each crew's PL. This setup was derived from the concept of legacy two-wire intercoms, when you used a source assign panel to route these party lines to each line of users. With VCOM, you can set this up remotely by assigning a selector for the PL on each user's VCP.