The model pool is the set of models currently hold on MEMORY (and not necessarily on the hard disk).
They are normally referred to the same tomogram, in particular the one currently on edition by the catalogue system. As a safety check, each time you enter a tomogram browser, if a model pool is detected on memory you will be asked if you want to delete it (because it was defined on a different tomogram) or keep it (because maybe you are just viewing a different version of the same tomogram).
From all the models in the model pool, at any given time only one (or none) is consedered to be active. Single model edition commands (as adding or deleting points) performed on any volume browser will always affect only the active model. The different browsers should show in the model area which model number is currently active.
Model pool and Catalogue
Models in the model pool can be edited (add/delete points, etc). Those editions will not be kept in disk automatically. You need to make certain that you save the changes into the catalogue before leaving a model pool. You can save into the catalogue all the models in the pool globally, or save individually only the ones that you want to store.
Conversely, you can import from the catalogue (i.e., from disk) any number of catalogued models referred to the tomogram currently in edition by the catalogue system.