Hi,

Custom report types are one of the best features that Salesforce has to offer.
A report type defines the set of records and fields available to a report based on the relationships between a primary object and its related objects.
There are a number of things within Salesforce that can only be reported on when using a Custom Report Type as the basis for a Custom Report.  They include:

    Reports with or without Dashboard Components
    Dashboards and their Dashboard Components
    Knowledge (by creating Custom Report Types on each Article Type)
    Reporting on Chatter
        Chatter Activity
        Groups
        Hashtags
        Users with or without Followers
        Users with or without Chatter Activities
    Domains and Domain Sites
    Mobile Sessions
    Ideas
    Data.com (Note there is a free AppExchange App containing a few Data.com Dashboards)
    Identity Provider Event Log
        Since no one knows what this is: The identity provider event log records both problems and successes with inbound SAML authentication requests from another app provider, and outbound SAML responses when Salesforce is acting as an identity provider

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Thank You