Regarding alerts/alert tasks related actions:
DISMISS: You can only "dismiss" a none-task alert, they are triggered as notifications on an account for you to review and decide if you need to take action on them (if so you convert them into a task). Otherwise you "dismiss" (remove) them from the alert dashboard so you don't have to pay attention to them anymore unless you choose to show "dismissed" non-task alerts via the alert view filter.
There is also a "dismiss" button when you need to edit/modify a task's detail, the dismiss button shown on the task details pop up window is simply for closing that window after you've done editing the task detail.
CLOSE: You can only "Close" an alert task - alerts that have been converted into a task so the CSMs can follow up and complete for an account. Closing an alert task means that the CSMs has completed all the action items (specified as playbook subtasks attached to that alert task) they need to deal with the alerted account situation. When you close an alert task, you need to specify if this situation has been dealt with successfully, unsuccessfully or false alarm. This feedback will be reported for manager review in the Team performance dashboard - tasks page for alert tasks.
Natero reports time spent in dealing with a task, so the "close" action sends the close time of a task for Natero to calculate task completion results in aggregate.
DELETE: "Delete" a task will remove it completely from Natero as if it never exists. If a task is irrelevant or wrongly generated for an account, you can delete it to clean up your task list and not have it reflected in the team-performance task reporting.