 Balanced indicator function of a finite subset

For nonempty finite set $I$ and $A\subseteq I$, let $\alpha =|A|/|I|$ be the density of a finite subset. The balanced indicator function $f=1_A-\alpha 1_I$, extended by zero to an ambient finite group, has sum zero and vanishing zero Fourier coefficient. The sum of $f$ on a subset $P\subseteq I$ is $|A\cap P|-\alpha |P|$, measuring the excess density of a finite subset there.