Hot-tap is a term used for an assembly that can be installed, inserted, and removed from a full pipe without process shut-down. In the case of the 2552, the installation procedure of the hot-tap sensor would look something like this:
Weld a threadolet (weldolet) fitting onto the pipe; connect a nipple and install the ball valve
Use a special hot-tap tool to make a hole in the pipe; close off the ball valve as soon as the hot-top tool is removed.
Attach the sensor to the ball valve, open the valve, and insert the sensor