The solve statement is optional and can be used only once. If no solve statement is used, one is assumed at the end of the model section.
The solve statement causes solving the model, i.e. computing numeric values of all model variables. This allows using variables in statements below the solve statement in the same way as if they were numeric parameters.
Note that variable, constraint, and objective statements cannot be used below the solve statement, i.e. all principal components of the model must be described above the solve statement.