l, u

Stereodescriptors of diastereoisomers with two chirality elements. When the two chirality elements are both R or both S the molecular entity is described as having an l relationship (l = like), when one chirality element is R and the other S the molecular entity is described as having a u relationship (u = unlike). The notation can be extended to molecules with more than two chirality elements whereby the l,u relationships of the chiral elements are considered in pairs.
Source:
PAC, 1996, 68, 2193 (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2211