Tapered threads form a seal by themselves. When tightening the threads to the proper torque, the threads tighten against each other and form a seal. Often, Teflon® tape or liquid paste pipe sealant is used to help the seal form properly.
Straight threads do not create a seal. Instead, they bear the load of the pressure and strengthen the connection. A gasket or O-ring is needed for the seal. On SAE fittings, the connection also has a cone shaped tip and is connected to a mating cone to provide the seal.
With APG pressure sensors, tapered connections are most common under 10K PSI. For anything greater than 10K PSI, we usually use the 9/16”straight thread with a cone.