According to King Arthur Flour, soft flour is made from soft wheat, which contains less protein than hard wheat varieties. Protein produces gluten when agitated, so soft wheat creates a less glutinous batter or dough than other varieties.
The amount of gluten in a baked product can drastically alter its texture. Crusty breads and pizza dough require hard wheat, which contains a lot of gluten that traps carbon dioxide bubbles, resulting in a crisp and chewy texture. Soft flour is better for cakes and cookies to produce tender, crumbly baked goods. All-purpose flour has a moderate protein content, somewhere between hard and soft flours in its gluten production.