The terms "Potential Free" or "Voltage Free" are engineering terms for "The module has relay contact outputs only", and it doesn't provide power for anything, or supervise anything (check continuity of any wires). It also doesn't provide power or voltage to activate anything.
What the engineer wants is a "Form C Relay". A Form C Relay has a Normally Open (N. O.) contact, a Normally Closed (N. C.) contact, and a Common (Comm or C.) contact. A control relay module on an addressable fire alarm system has a Form C Relay output.
By using the words "Third Party Interfacing", the engineer wants to turn on or off other equipment (not fire alarm equipment) using control signals from the fire alarm system. This other equipment includes things like air handlers, smoke dampers, elevators, security systems, etc.
The relay is really just a switch. Inside the switch is an electromagnet. When the electromagnet inside the relay is turned on, the switch inside the relay is turned on or turned off.
There is no electrical connection between the relay's electromagnet and the For C Relay contacts.