A Standing Arrangement
ことになっている describes a rule, custom or schedule that is already in force. Nobody is deciding it now.
- 会議は毎週月曜ということになっています。(The meeting is set for every Monday.)
Against ことになる
The ている is the whole difference:
| Pattern | Sense |
|---|---|
| ことになる | it has been decided — the moment of decision |
| ことになっている | it stands decided — the rule as it is now |
転勤することになりました announces news. 九時に始まることになっています states the standing rule.
Its Social Use
Because it presents a rule as impersonal, it is the polite way to refuse: the constraint belongs to the system, not to you.
- ここでは撮影できないことになっております。(Photography isn't permitted here.)
Nobody is being blamed and nobody is exercising authority, which is exactly why staff use it.
The Negative
ないことになっている states a prohibition the same way: 持ち込まないことになっています.
