It's all to do with the protein level of the flour. Cake flour is made from soft wheat that is bleached to increase it's ability to absorb liquid, sugar and fats.
Most substitutions are either
* 1 c cake flour = 12-14 tb plain flour + 4-2 tb cornstarch or other low gluten starch] OR
* 1 c cake flour = 7/8 c plain flour.
Description of sponge flour: ' milled from specially selected soft wheats producing an extremely fine flour ideal for sponge making. It can absorb more moisture and sugar than standard flour and will produce a very light, soft texture.'
If it is Self-Raising sponge flour, then it can be substitued for Self-Raising cake flour Or plain Cake flour and the leaveners in the recipe.
Generally, protein levels are [BUT they can fluctuate wildly due to locational, seasonal and $$$ factors]:
• Plain Flour……9%-13%
• Cake flour……………6%-10%
• Pastry flour…………..8%-11%
• Self-Raising flour……..8%-11.5%