Both usage options are sensible. If you have a really heavy flow on some days - e.g. at the beginning of your period - and you often have "accidents" (leaks and stains) with other hygiene products (tampons, pads, cups...), it is advisable to use the panties from Sisters Republic in addition to these other products. This way, you have a 100% leak guarantee and can finally experience your monthly cycle in a relaxed manner.
If you do not have a very heavy menstrual flow, you can definitely use the Sisters Republic Slip instead of your usual feminine hygiene products. However, you should choose the absorbency of your slip ("Light", "Medium", "Super") according to your current flow strength (light, medium, heavy) and adhere to the usage times specified below or, better yet, rely on your feeling and change the slip as soon as you feel the first moisture: