The model frame is produced by mvmeta when fitting the meta-analytical model, and stored in the mvmeta object if argument model=TRUE. Alternatively, the model frame is directly returned by mvmeta with argument method="model.frame". The method function model.frame simply extracts the saved model frame if available, or otherwise evaluates a call to mvmeta when method="model.frame".
The method function model.matrix extracts the design matrix from a fitted meta-analytical model. It first extract the model frame by calling model.frame, and then passes the call to the default method.
These methods functions are similar to those provided for regression objects lm and lm.