Self-rising cake flour is cake flour to which baking powder has been added.
It is used mostly in British recipes.
Substitutes
For self-rising cake flour, per cup substitute 1 ½ teaspoons of baking powder plus (1 cup of cake flour or 1 cup Canadian all-purpose flour or ⅞ cup American all-purpose or British plain flour).
Or per 5 oz / 140g called for, substitute 5 oz / 140g flour plus 1 ½ teaspoons of baking powder.