Three terminal LED pilot. What's the matter?
Industrial control panels often have a lamp test feature to check that all lamps are functional. This is less important nowadays with the switchover to LEDs which have a much longer lifespan than incandescent bulbs.

One of the problems to be dealt with is prevention of backfeeds from the test circuit. For example, if the lamp is connected in parallel with a relay then some means of preventing the relay from being energised during lamp test must be employed. On DC lamps a couple of diodes can usually provide a solution. It appears that in your product the diodes have been included in the indicator housing. Nice!







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Likely internal indicator circuit and sample external circuit including test switch.

I expect that you should find internal diodes to prevent backfeed from the test circuit into the control circuits and vice versa. I would also be quite confident that SIG and TEST terminals could be swapped without a problem.