ag.algebraic geometry – Semi-stable sheaves on quadric surface

In this paper, Stable sheaves on a smooth quadric surface with linear Hilbert bipolynomials(E. Ballico and S.Huh), I have a question.

On the right down of the page 2 of this paper,

For every semistable 1-dimensional sheaf $F$ with $chi_F(x, y)=mx+ny+t$, define $C_F:=mathrm {Supp}(F)$ to be its scheme-theoretic support and then we have $C_F in |O_Q(n, m)|$.

Question : In this sentence, how we conclude that $C_F in |O_Q(n, m)|$?