The latter means that if a set of droplets, or containers, or servers, or services should have access to a different set of information than another, host them separately. You wouldn't host your accounting software for your business in the same account as the service you offer customers, as an obvious example.
But also, you might note that a security domain changes based on who needs access to the droplets (etc) within it.
Too many accounts, though, will prove difficult to manage and that in turn will probably lower security.
If you have more than 3 or 4 security domains you're probably dividing too finely.
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