When using the plugin Batch classification of projects (in older versions plugin "Modify Order Number and Type Code") you can assign desired variables to a selected class attribute successively.

Select a variable. The plugin goes through the entire structure and at projects containing the variable, the classification will be assigned.

Afterwards a listing shows at which projects the assignment was successful and at which not. Now you can use another variable for the assignment as long as all projects are classified.

Details on this can be found under Section 5.13.6, “ Batch classification of projects ”.