  We study $f$-vectors, which are the maximal degree vectors of $F$-polynomials
in cluster algebra theory. For a cluster algebra is of finite type, we find
that positive $f$-vectors correspond with $d$-vectors, which are exponent
vectors of denominators of cluster variables. Furthermore, using this
correspondence and properties of $d$-vectors, we prove that cluster variables
in a cluster are uniquely determined by their $f$-vectors when the cluster
algebra is of finite type or rank $2$.
