Saltcorn has power ability to extend its functionality via:
Plugins (also named as Modules),
Themes are special type of Modules that install UI themes.
Packs are collections of defined tables and views, along with the plug-ins needed to support them. You can use them to quickly get started with an application by adding bundles of functionality and then modifying it.
Whereas modules extend Saltcorn with functionality that you cannot define to the user interface, packs only replicate functionality that you could define yourself. They are only there to save you time and not to extend with new functionality.