Under Manage → Email Manager, when selecting any email template/notification for editing, you have multiple pre-defined variables to choose from on the left-hand side.
Some come with and without SYS in front of them - for example, you can choose RESOURCE_URL or SYS_RESOURCE_URL.
The difference is, that non-SYS variables can be translated through Manage → Localization to different languages (if you have Localization enabled for your application).
Sys variants always use Piano’s pre-defined translations in case you have Localization enabled.