By using Proloquo2Go's VocaPriority levels (Primary, Secondary and Storage), you can organize the vocabulary by frequency of use. Buttons that are frequently used should be placed in the Primary level, while buttons that are used occasionally should be placed in the Secondary level. Buttons that are never used but which you do not want to delete permanently should be placed in Storage.
Storage is not visible during regular use, just while in Edit Mode. It contains all the buttons that are not used by the user or for which the user is not yet ready. As a user becomes more proficient, you can promote buttons out of Storage to the appropriate level.
For exact instructions on how to use VocaPriority, please see this tutorial.