I am reading about distributions in the context of differential geometry.

A distribution $S$ of dimension $r$ on a manifold $M$ is an assignment to each point $p \in M$ of an $r$-dimensional subspace $S_p$ of $T_pM$.

(1) Is this $r$ called rank of the distribution?

Further, I introduce a closed $3$-form $\alpha$ on $M$, and consider the distribution $\ker \alpha := \{X \in TM: \alpha(X,{}\cdot{},{}\cdot{}) = 0\}$. I read that if $\ker \alpha$ is of constant rank and has closed leaves, then I can consider the quotient $M/\ker \alpha$ with a structure induced by $\alpha$.

(2) I cannot really understand the last sentence above. Is the condition 'constant rank' necessary to construct the quotient $M/\ker \alpha$? What does 'closed leaves' mean?

Any help about (1) or (2) is appreciated. Thanks.