The (R) enantiomer of a compound has a specific rotation of $+30.0$ under certain conditions. Under identical conditions the specific rotation of the (S) enantiomer is

Enantiomers rotate plane-polarised light by the same magnitude but in opposite directions under the same conditions. Therefore the (S) enantiomer has $[\alpha ]=-30.0$. The (R)/(S) label has no fixed relation with the sign of rotation, but here the (R) sign is given.