Divisor of two relatively prime

Question

Let $X=\mathbb{P}^1$, let $f,g\in k[t]$ relatively prime, where $t=X_1/X_2$, two coordinates. Then what can we say $\operatorname{div}(f/g)$? I can just tell that $\operatorname{div}(f/g)=\operatorname{div}(f)-\operatorname{div}(g)$. Is there any further result more than that ?