(+) and (-):

The  assignment  of  an  asymmetric  center  like  R  or  S  has  nothing  to  do  with any direction the molecule rotates plane-polarized light. Optical rotation can just only be ascertained experimentally. Through convention, molecules that rotate plane-polarized light clockwise are described as (+) or d.  Molecules that rotate plane-polarized light counterclockwise are identified as (-) or l. The R enantiomer of lactic acid is found to rotate plane-polarized light counterclockwise and thus this molecule is identified as (R)-(-)-lactic acid.