As documented, in a NAS Auto Operator rule, the action mode "Trigger" restricts the number of Action types available. (ie. it does not include the mode repost).
The reason for this is that the trigger is just a state and not an alarm.
The Actions types for Trigger mode are limited to the types that are not supposed to contain alarm data.
Available with "on trigger" action mode:
command, EMAIL, new_alarm, post_message, script, SMS.
Not available with "on trigger" action mode:
escalate_level, repost, set visibility.
For the Action types that are both available for on_trigger and for other action modes, the displayed fields may differ depending on the action mode selected.
For example, while selecting "On trigger", the EMAIL action mode requires a subject and message body to be defined by the user.
When selecting the rest of the action modes instead, the Subject and the Message Body are greyed out as the information in that case would come directly from an alarm, and not from a trigger, which does not contain alarm data.