One hundred grams of flour appears often in European recipes and small cakes. It is about five-sixths of a US cup, not a full cup. That awkward fraction is a good reason to use a scale when the original recipe is written by weight.
For all-purpose flour, the calculation is 100 ÷ 120 = 0.83 cups. If you use cups, fill each measure the same way from start to finish.
Do not change to a lighter flour type just to reach a simpler cup fraction. Select the flour named in the recipe because protein level and behavior matter as much as volume.