Which user port ground pin for project?

[rwv01]

Hi all.
I'm building a user port interface from an old schematic I found in a disk image.
It's for the C64. A ham radio interface for de-coding FAXes and Slow-Scan TV images https://en.wikipedia.org/wiki/Slow-scan_television.

Anyway, If someone can help me, I just need to know which "pin" to use on the user port for the ground for this circuit.
Protective ground? Signal ground? Does it matter?
Also, the audio-in should be connected to the same ground, correct?
And what does "UI T (?)" mean at the Flag 2 connection?

The circuit

If you can help, thanks!

[srowe]

Anyway, If someone can help me, I just need to know which "pin" to use on the user port for the ground for this circuit.
Protective ground? Signal ground? Does it matter?

The ground pins on the user port are all connected to the same ground plane. Those separate designations are used by the RS-232 standard.