What is the reference frame used when sending a NAV_TAKEOFF command via CommandLong?

I am trying to have a PX4 Autopilot drone takeoff using Mavros and it’s set_takeoff service. Based on the mavros source code, this service sends a CommandLong message to PX4.
When takeoff occurs the drone isn’t flying as high as I expected. I believe this is due to the frame of reference being used for the altitude value. I know the frame can be sent with CommandInt but I would prefer to use the Mavros interface which uses CommandLong.

What frame is being used in the PX4 Autopilot when it receives a CommandLong message with cmd NAV_TAKEOFF? It’s not jumping out to me in the source code.