https://www.servicenow.com/community/it-service-management-forum/email-notification/m-p/695221 Reference:
When configuring the content of an Email Notification, the following syntax should be used to reference the properties of an event triggering the Notification:
event.<propertyname>.ThisisthecorrectsyntaxtoaccessthepropertiesoftheeventrecordthattriggeredtheEmailNotification,suchasevent.name,event.parm1,orevent.parm2.Forexample,{event.parm1} will display the value of the first parameter of the event.
The following syntaxes are not correct for referencing the properties of an event triggering the Notification:
current.<propertyname>.Thisisthesyntaxtoaccessthepropertiesofthecurrentrecordthatisassociatedwiththeevent,suchascurrent.number,current.shortdescription,orcurrent.state.Forexample,{current.short_description} will display the short description of the current record.
${property name>.getDisplayValue()}. This is the syntax to access the display value of a property of the current record, such as current.state.getDisplayValue(), current.assigned_to.getDisplayValue(), or current.category.getDisplayValue(). For example, current.state.getDisplayValue() will display the state of the current record in a human-readable format, such as New, In Progress, or Closed.
${gs.<property name>}. This is the syntax to access the properties of the GlideSystem (gs) object, which provides methods for performing system operations, such as gs.now(), gs.getUserID(), or gs.getProperty(). For example, gs.now() will display the current date and time of the system. Reference: Email Notifications, Email Notification Variables

https://docs.servicenow.com/bundle/tokyo-application-development/page/build/applications/task/set-RCA-level.html

A Display Business Rule has access to the current, g_scratchpad, and GlideSystem objects, but not the previous object. The previous object is only available to Before Business Rules4. Reference: Business Rule API
https://docs.servicenow.com/bundle/tokyo-application-development/page/script/business-rules/concept/c_BusinessRules.html

A possible reason to build a custom app is to fulfill a specific use case on internal processes. For example, you may want to digitize a manual process that is not covered by an existing ServiceNow solution, such as managing inventory, tracking expenses, or scheduling events. Building a custom app on the Now Platform can help you automate workflows, improve data quality, and provide better user experiences. The other options are not valid reasons to build a custom app. To avoid using a code repository like GitHub or GitLab is not a reason to build a custom app, as you can still use source control integration with your custom app development. To create a custom integration for a 3rd party system is not a reason to build a custom app, as you can use integration tools such as IntegrationHub or REST APIs to connect with external systems without creating an app. To replace ServiceNow base tables is not a reason to build a custom app, as it is not recommended to modify or delete base tables that are essential for ServiceNow functionality. Reference: Build Custom Apps in ServiceNow - eBook