A onetime is a job that fires once and is then deleted by the scheduler.
A onetime event can either file now, at a specific time or on a specific date at a specific time.

Probably the most common use of this type of script is to have it run during setup of a larger script.
Often you will want to do things like setup environment variables, map drives, etc..

Very often you will use a run now script like the following:

# Onetime event, will fire once now then be deleted
{
-start
-stop
-command Defrag /s /f
}

Sometimes you will want your script to fire at a particular time regardless of the date (at 2:33 PM any day it is run.)

# Onetime event, will fire once at 2:33 PM then be deleted
{
-start -time 2:33:00 PM
-stop 10/12/1998 2:33:00 PM
-command Defrag /s /f
}

You can also setup a script to fire on a specified date at a specific time.
This has the built-in property of not being reusable after the specified date has passed.

# Onetime event, will fire once on 10/12/1998 at 2:33:00 PM then be deleted
{
-start 10/12/1998 2:33:00 PM
-stop 10/12/1998 2:33:00 PM
-command Defrag /s /f
}