It is a measure of strength. It is an expression of grams per liter. The strength of the vinegar determines the effectiveness for an application. For simple dissolution of fats and such a weaker solution woud be appropriate. For disinfectant properties the pH makes a differentce, needing at a pH of 4 or lower. When you dilute vinegar you raise its pH. A vinegar of 5% may have a pH as low as 2.4 for distilled white to as high as 3.5 for apple cider. Most vinegar cleaning solution recipes don't mention pH thinking people can't afford a $30 meter of $3 test strips. Vinegar of 10% becomes 5% when you double the volume with water. The pH will raise accordingly but not by simple math like the strength.
The 5% standard is used for home recipes because, as a "standard", dilutions can be done by volume to achieve a desired pH.
