Zo less than or equal to 500 ohms?

I have a tally light pin coming out of a camera that I would like to use to trigger a transistor. On the pinout sheet it lists the pins specs at Zo less than or equal to 500 ohms. I am not sure what this means, or how to use it to select a transistor. I would like to have this pin be the base and the collector be an external 12v. I understand how a transistor works, but not how to match one to my needs. If more info is needed please let me know.