Basement size?

What percentage of far (covered area) is permissible for basement?
I couldn't understand last part of the below by-laws
8.0(b): the areas of basement shall not be counted into far provided it has access/approach from the ground floor. In case it has exclusive external approach and has no internal access from the floor, the area will be counted into total far, but it shall not exceed 25% of the permissible ground coverage.

The last sentence says
"but it shall not exceed 25% of the permissible ground coverage".
Is this sentence regarding basement size or external stares size because only stairs are discussed in the same clause.