You cannot limit specific choices in the Priority field, but there is a method to work around this. It involves using the AutoField functionality to determine the Priority based on new fields called Urgency and Impact.
Urgency could have values like High/Medium/Low. I prefer something more specific like Down/Affected/Service Request. I have also seen Project thrown in there.
Impact could have values like High/Medium/Low. I prefer Company / Department / Individual. You may have to adjust these based on your organizational structure. I have also seen variations such as VIP and Store or Location.
You could make Urgency publicly available to customers, while keeping Impact agent-only defaulting to Individual.
For someone to select P1 in your case, they would have to state the Entire Company is Down because... one persons needs an ergonomic keyboard. This would be very odd and hard to defend. Improving Prioritization using Impact and Urgency is less prone to accidents of misunderstanding. Moreover, it is more consistent with industry practices such as ITIL.
you can use thisMRLocalDefs
$HIDDEN_PRIORITIES ="4 5 6";
#These priorities must benumeric and will not be available on the create new issue page, and if anissue is already in this priority then it cannot be changed on the Editpage. Escalations and incoming email WILL be able to change/set thesepriorities though.
// this will be applied forall agents
FYI, MRlocalDefs are notsupported by BMC.
If you don't want to useAutofield like what Gregory suggested... you can create an internal priorityfield (with the same values of the initial priority but without P1) that willbe visible just for the service desk group and change the role of this group sothey can't edit priority... than create an autofield that will change thepriority depending on the internal priority...