Is this line Instrumentation Amplifier's rail or not?
Think of it as the reference for the diff amp.  The amplifier will take the difference between its two inputs, multiply that by some fixed gain, then add that to a reference to make a single ended signal.  That's what diff amps do.  The single-ended output has to be relative to something.  AI GND is that something in your schematic.

It is also possible that this same input is the negative power supply input for the diff amp.  Your schematic doesn't make that clear one way or another.  The datasheet for the part, though, surely goes into detail on all this.