Halmos and Givant state p. 379 of their Introduction to Boolean Algebra:
The class of Baire sets of a σ-space is a σ-algebra, by definition. The subclass of meager Baire sets is naturally a σ-ideal in that algebra. It turns out that Baire sets are almost clopen sets in the sense that each Baire set differs symmetrically from a uniquely determined clopen set by a meager Baire set.
Let $X$ be a $\sigma$-space, $B(X)$ the $\sigma$-field of Baire subsets of $X$, and $M$ the $\sigma$-ideal of meagre subsets of $B(X)$. What does $M$ contain exactly?
Clearly, it contains the empty set. It also contains the boundaries of open-closed sets (which are the empty set), the boundaries of open Baire sets, the boundaries of closed Baire set, the boundaries of neither open nor closed Baire sets, the Cantor set (which is a set of boundary points). Anything else? In particular, anything else but boundaries or sets of boundary points?