https://circuitindustryjournal.wordpress.com/2013/11/25/why-you-might-use-an-alternistor/Another name for these is the three-quadrant TRIAC. They are made to only work in quadrants 1 through 3 but not 4. It turns off more quickly than other models as well. They don’t need snubbers due to this limitation and are often called “SNUBBERLESS”.
A BTA24-600B is a triac, denoted by the B suffix. A BTA24-600BW is an Alternistor, as would be a CW suffix (snubberless). The datasheet still references "Triac" for all of them.