RSSI display on HUD

I recently bring out “RSSI” from the Status section to display in HUD. During bench testing, there is a RSSI value shown, but not always. Sometimes it shows zero even my Tx is ON. The RSSI value appears randomly in time.
I don’t have a receiver with RSSI capability. My FC is Pixhawk, and the MP and the arducopter firmware are all up to date.
Pls share your opinion.

rssi or rxrssi?
one if for telem radio the other receiver

Thank you Michael.
It is rssi I bring up to display on HUD, not rxrssi.
I then brought up the rxrssi to HUD, and when I power up everything incl
Tx, with the quad connected to MP vis telemetry, the rxrssi shows zero, and
not change. This is expected as my Rx has no rssi capability. But the rssi
for the telemetry shows 201, and when I moved the quad away from the GCS
MP, there is no change in the signal strength figure, still showing at 201.
Pls advise what makes the figure change.

Appreciate your help.