In page 4, section 3 of this paper, they define a set $Q$ as a set of subsets of $Z$ "such that their union spans $Z$". Can someone please explain this part? I don't understand what this condition means? Say $Z$ is defined as follows: $$Z= \left\{h_{1},\space h_{2}, \space h_{3} \right\} $$
Then how would $Q$ be defined, "such that the union of its members spans the set $Z$"? Is there even a unique $Q$?