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Answer: The "Custom Immediate" schedule type is a custom schedule where the events are defined by waiting a certain number of minutes after the last event. This use case has caused some confusion, especially when used with a repeating schedule, so it's use is discouraged and support for this use case is limited. As a result, you can create new alerts of this type but there are some parts of it that will not be editable. You can still edit the message content that is sent, however. If you really need a custom schedule type, consider using the Custom Daily schedule which is a much more robust custom schedule type and allows editing of events.