Those two features are based on near the same user interface.
You can create an element, give it a name and use it through the dedicated menu.
In that name, a character preceded by a '&' can be used as keyboard shortcut.
The first time you started Siren some examples have been created in the "Favourites" and "Tools" menu. They may perhaps be useful.
To know how to retrieve this data after changing of Siren version, follow this link.
The "Favourites" menu will enable to keep and reuse your preferred renaming expressions.
Activating one of them will copy the associated expression in the "Expression" text field and recompute the "New names" of the selected files.
A tool corresponds to a system command which syntax is the same as the renaming expression ones : variables, modifiers ... are allowed.
When "invoked", the system command associated to the name is executed on
each highlighted file.
Before execution, the command is evaluated the same way as what is done to compute a "New name".
Therefore it can contain information specific to each treated file.