From an existing Appian application, I saw there is a User record type which was created and set a source type "System". but I could not find the "system " source option in my Appian Cloud Community edition? is there anything setting to turn on this source type?
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This is a system record type that points to the system users, you cannot change it.
It is the record of users in the system, that's why it is of system type. In my knowledge we cannot create a record type having source other than Database, webservices, salesforce ,and process model.