Appendixes | Report Option Descriptions | Model baseline as:

Model baseline as:
Use the radio buttons to specify how you want to use the baseline measurements as the standard of comparison, in a repeated-measures ANOVA model.
Your options include Covariate or Response.
When you select covariate, the first observation for each subject is removed. The deleted values are then averaged. This average value is entered as the first fixed effect and used as a covariate in the repeated-measures anova model.
When you select response, the baseline measurements are treated the same as the trial measurements in the repeated-measures anova model.
For Additional Information
Refer to SAS for Mixed Models1 for additional details about Repeated Measures anova.

Littell, R.C., G.A. Milliken, W.W.Stroup, R.D. Wolfinger, and O. Schabenberger. 2006. SAS for Mixed Models, Second Edition Cary, NC: SAS Institute Inc..