**Unrestricted model**:
A mixed model (i.e., with random and fixed factors) is termed unrestricted if a
random factor is allowed to have fixed cross factors amongst its components of
variation estimated in the population. This freedom influences the choice of
error Mean Square for random effects. It is generally used for unbalanced
designs analysed with a General Linear Model.
Balanced and orthogonal
designs can be analysed with a restricted 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.

http://www.southampton.ac.uk/~cpd/anovas/datasets/