The custom data source property enables you to select a microflow as data source for a reference selector. This means that the reference selector, instead of executing an XPath query, calls a microflow to retrieve a list of objects to show in its dropdown list.

The return type of the microflow should be a list of objects of the same type as the type of object the reference selector is used for. So, if you for example use a reference selector to select a System.User object the microflow used as custom data source should return a list of System.User objects.

The custom data source lets you select a microflow that returns a list of objects that become available for selection in that reference selector. An example of usage for this would be if you have multiple relations between 2 objects and you are using 1 specific relation between these objects in the reference selector, but you want all the objects to be available for selection.

For example: You have an Account object that has multiple Folder objects. There are multiple relations between the Account and Folder object, such as InboxFolder, OutboxFolder, etc. You create a reference selector that lets the user select his Inbox folder, by using this specific relation between these objects. But you want all folders to be available for selection in the reference selector. So you create a microflow that returns all folders belonging to this Account and use this as a custom data source.