When should I choose to make a Data Layer Selectable or not?
Using the Data Layers > Data Layers admin page Incidents tab you can specify if a Data Layer is Selectable within any specific Incident project. Choosing whether a Data Layer is Selectable will utlimately depend on the type of reporting for your Incident map.
For example if the Graffiti Incident map has the Selection Type set as Free then you will likely want your Graffiti Data Layer not to be Selectable. This means that the Graffiti Data Layer is simply loaded into the Graffiti Incident map for context only, allowing the users to click on an existing feature to view the Information Bubble (not to record an incident). Users will then click anywhere else in the map to record a new incident at a Free location.