How are unidirectional sealing butterfly valves and bidirectional sealing butterfly valves distinguished?

One-way sealing butterfly valve:

Unidirectional sealing butterfly valves feature a disc that faces the flow direction of the medium when closed. The medium flows in only one direction, and the valve body must display an arrow indicating the flow direction. During installation, pay attention to the direction of medium flow.

Double-sealed butterfly valve:

Bidirectional sealing butterfly valves feature a disc that can face either direction relative to the flow of the medium. Installation does not require consideration of the medium flow direction, and the valve body lacks arrows indicating flow direction. The stem of a bidirectional sealing butterfly valve experiences greater forces than that of a unidirectional sealing butterfly valve. In design, for butterfly valves of the same diameter and pressure rating, the stem diameter of a bidirectional sealing butterfly valve is larger than that of a unidirectional sealing butterfly valve.