GLM, GLZ, GRM, PLS, and GDA Syntax - Keyword Surface

SURFACE [=] Continuous predictor (covariates) list;

Example
SURFACE = TIME DEGREES;

Optional keyword, currently not used in computations; specify the continuous predictors (covariates, specified via the COVARIATE keyword) that are used in a response surface design (as specified via the DESIGN keyword).

Applies to
GLM, GLZ, GRM, PLS, and GDA.