ということは and ということだ: inference and interpretation
Compare drawing a conclusion from a fact with explaining what information means, while preserving the degree of uncertainty.
Use ということは to make a fact the starting point
電気が消えているということは、店はもう閉まったのでしょう。 means that the lights being off probably means the store has already closed. The observed state of the lights becomes the starting point for an inference about the store.
An established fact and an inferred conclusion need not have the same certainty. でしょう leaves the conclusion tentative, so do not turn the lights being off into absolute certainty that the store has closed.
Use ということだ to explain the meaning
この印は、ここで写真を撮ってはいけないということです。 means that the sign indicates photography is prohibited here. The sentence spells out the message conveyed by the sign and presents that interpretation at the end.
つまり、申請には本人の署名が必要だということです。 means that, in other words, the applicant's own signature is required for the application. つまり makes the summarizing relationship clear and helps distinguish it from a context that merely relays something heard.
Check the content and strength of the conclusion
彼が来ないということは、予定が変わったのかもしれません。 infers from his not coming that the plans may have changed. Preserve both the negative in 来ない and the possibility expressed by かもしれない.
ということだ can also relay received information in other contexts, but the sign and つまり examples in this guide explain meaning. Notice the structure: は makes the preceding content a topic leading into a judgment, while だ presents the interpretation at the end of the sentence.
Does 彼が来ないということは、予定が変わったのかもしれません。 state that a change of plans has been confirmed?
Show the answer and why
No. It infers a possible change of plans from his not coming. Because かもしれません marks uncertainty, the changed plans must not be presented as an established fact.