Restricted model: A mixed model (i.e., with random and fixed factors) is termed restricted if a random factor is not allowed to have fixed cross factors amongst its components of variation estimated in the population. This restriction influences the choice of error Mean Square for random effects. It is generally used for balanced and orthogonal designs; non-orthogonal designs requiring analysis by General Linear Model use the unrestricted model.
Doncaster, C. P. & Davey, A. J. H. (2007) Analysis of Variance and Covariance: How to Choose and Construct Models for the Life Sciences. Cambridge: Cambridge University Press.