Hi @akin9 ,

You can create two ACL on incident table on urgency and Impact field with write operation with condition state is onhold

and add role ext_itil .
Then ext_itil user also able to edit urgency and Impact.

Note: No need to write any acl or rule over priority field , priority field auto calculate when impact and urgency changed.

Please mark the answer correct/helpful based on Impact.
Regards,

Heena Sharma