Custom site variables can be declared in the Settings under Custom site variables.
Custom page variables can be declared in the Settings under Custom page variables.
The aim of custom site variables is to collect and display a key piece of information that can be found on several pages of your site, and is applicable to the entire visit (e.g. language used, user profile, etc.).
On the other hand, custom page variables are used to gather information from specific pages that is only applicable to said pages (e.g. forms).
Upon declaring these variables, it is necessary to select a format for the data. This cannot be changed later, so if you have any doubts the safest option is to use the "text" format. This will allow for any kind of string.