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For full context:

[![enter image description
here](https://i.stack.imgur.com/9WIWd.png)](https://i.stack.imgur.com/9WIWd.png)

> 僕たちは一つだけ決めていることがある。 それはよる10時を過ぎたら、自分の部屋で過ごすということだ.

I think this ということだ expresses "means". "Concerning this...it means (that)".
I'm asking for confirmation because my textbook also teaches ということだ as a means
to express hearsay or to summarize what the partner in dialogue just said.

My attempt at translation: "Concerning this, it means that when it is past 10
o'clock in the evening, I/we (?) pass time in my/our own room(s)."
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grammar meaning expressions
What is the meaning of this ということだ
Look at the previous sentence for context:

> 僕たちは一つだけ決めていることがある。

"We just have one rule." So the ということだ nominalizes and raises the meta-level
of the sentence to make clear that it's referring to the rule, not just a
future or habitual action.

I believe this is covered in detail in _A Dictionary of Intermediate Japanese
Grammar,_ if you happen to have that book.
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> to summarize what _the partner in dialogue_ just said.

Your textbook has to be edited soon, like "to summarize what ~~the partner in
dialogue~~ _has_ just _been_ said".

自分が長々としゃべったことを最後に「要するに〇〇〇〇ということです」と言って、要約する（＝ 簡単にまとめる）ことは普通に行います。

Also your textbook has to add the following definition to what is written
above: "to make clear what I/we declare/declared, decide/decided or the like".

"ということだ" in the text you posted corresponds to the latter definition edited.
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The phrase ということだ does have several idiomatic uses, like indicating hearsay
and paraphrasing, but it is not the case with this particular ということだ. Overall
meaning of the sentence will be clear by just adding up those of its component
words (if we treat ということ as one word having the function of nominalization,
that is). The basic structure of the sentence is almost as simple as that of
「私は学生だ。」("I am a student."):

それ(Topic Noun) + は(Topic Marker) + よる...ということ(Predicate Noun) + だ(Copula)

Translation:

> There is just one rule we agree upon. **It is that we spend time in our own
> respective rooms after 10 p.m.**

It's just that your sentence has a predicate noun with ということ, followed by だ,
and ということだ can have different interpretations in different cases, unlike 学生だ,
which always means _be a student_.
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