Council Chamber -- just like a Parliament. You pass a security check upon entering the whole building (Green Sector, diplomatic area) and can move freely in the public spaces within. To enter specific non-public rooms, you would need to pass an additional check or have a key.
A random Vorlon disguised as a refrigerator would not rush into the Council Chamber without anyone trying to stop it. A rather annoyed Delenn would. Not only would she have authorization, but she would also walk past any guards (remember the Grey Council) or stare at them until they would step out of her way.
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Personal quarters are governed by personal preferences. You can program who your computer will let it, how the doorbell works, how fast (and under which conditions) the door closes. The door has a neat set of sensors for recognizing people and determining their intentions, you have full control over your own computer (which can, to some degree, understand spoken language).
Example: business quarters. If the owner is at home, anyone may enter. No authorisation is needed to enter a shop. The door will open if a person stops at the door and faces it (unless occupied with other activities like a conversation).
Example: a diplomat's office, most personal quarters. You ring a doorbell and answer three questions: who are you, what do you want, why are you here.
Some persons may get in more easily: if the computer recognizes Vir, he will get into Londo's quarters. You can recognize a person via various means: visual examination, fingerprint, DNA.
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Reverend Dexter and Sheridan's quarters: dramatic license.
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"We are the universe, trying to figure itself out.
Unfortunately we as software lack any coherent documentation."
-- Delenn