Displacement from mass one to mass two

The vector $\mathbf{r}_{21}=\mathbf{r}_2-\mathbf{r}_1$. Its magnitude is the scalar separation.

Supplies both the distance that appears in the inverse-square factor and the line along which the force acts.

Only the magnitude $|\mathbf{r}_{21}|$ weakens the force; the direction of $\mathbf{r}_{21}$ aims it.