we cant define security for CDT. Does that means everyone can access CDT ?Or is there any way we can establish security for that
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Yes, CDT does not have any security but you can set security on Datastore level.
Except setting security on Datastore level , there is no other way ?
As far as i know , No . is there any specific requirement to add security for CDT?
There is no security for CDTs. But keep in mind that the CDT is only the definition of a data structure.