Hydrotest

You have not given us enough information to answer your question.

- Hydrotest vent/drain on what?

- Vessel? How big?

- Piping? How big?

- Where did you see these fittings called for? The client specifications? The project specification? On detailed drawings? Where?

what I see googling those terms are what we call weld-o-lets or theard-o-lets. These are forged fittings that allow you to put small 1/2" to 2" taps on lines that you can then use to put water into the the lines for testing. And for high points to get air out filling.