OTRS comes by default with five priority levels. If you'd like to customize this you can do so via the "Priority" option in the Admin console. When creating a customized list of priorities please keep in mind that they are sorted alphabetically in the priority selection box in the user interface. Also, OTRS orders tickets by internal database IDs in the QueueView. Please take this into account when customizing priorities.
Due to the nature of OTRS it is not possible to remove existing priorities, you can set them to "Invalid" though.
If a new priority was added or if a priority was changed, you might also want to modify some values in SysConfig:
Ticket:Core::Postmaster::PostmasterDefaultPriority - defines the default priority for all incoming email.
Ticket:Frontend::Agent:Ticket::ViewPhoneNew:Priority - defines the default priority in the New Phone Ticket screen for the agents.
Ticket:Frontend::Agent:Ticket::ViewEmailNew:Priority - defines the default priority in the New Email Ticket screen for the agents.
Ticket:Frontend::Customer:Ticket::ViewNew:PriorityDefault - defines the default priority in the New Ticket screen in the Customer frontend.