決定 and 確定: reaching a decision and finalizing information
Distinguish choosing a venue from finalizing attendance figures, without reading finalization as permanent impossibility of change.
Reaching a conclusion through discussion or choice
決定, read けってい, means deciding on a conclusion among possibilities. 話し合いの結果、会場を決定しました。 means that the venue was decided after discussion.
決定は来週行います。 says that the decision will be made next week. It schedules a decision rather than reporting one already made.
Moving from provisional to settled
確定, read かくてい, means that details or figures become firmly settled. 参加人数が確定しました。 says that the number of participants has been finalized.
仮の日程は出ていますが、まだ確定していません。 says that a provisional schedule is available but has not been finalized. Published information is not necessarily a final arrangement.
Reading the focus where the words overlap
日程を決定しました。 and 日程が確定しました。 can both indicate a settled schedule. The first emphasizes making the decision; the second emphasizes the schedule reaching a settled state.
A finalization notice does not linguistically rule out all later changes forever. If a finalized arrangement is subsequently changed, check the separate change notice.
Can the provisional schedule in 仮の日程は出ていますが、まだ確定していません。 be treated as final?
Show the answer and why
No. A provisional schedule has been provided, but the sentence explicitly says that it has not yet been finalized.