In appian developer course, they mentioned CDT is used for Record type with no sync
But in the next Leason , they mentioned CDT will be created automatically once the Action is created for record type with Datasync enabled.
where we use CDT ? for non synced record type or synced record type
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In the 2nd screenshot you attached, except the first line ('This is perhaps the simplest and most intuitive way to create a CDT'), nowhere the text implies about CDT but its all about generating record action. I can see why it can be confusing as the first statement honestly is not making much sense to me here!
In simple distinction, CDT is used for non synced record types.
When data sync is enabled for a record and you generate actions, then interfaces and process models are generated automatically. CDTs are not generated in this case.
docs.appian.com/.../record-actions.html