This IR P85 tax form is used by people leaving the UK. It is used to notify HMRC that you are leaving and the tax basis on which you are leaving.
The official page for this form is on the HMRC website below:
Given the unstable HMRC 'rules' on UK non-residency you should take advice on completing this form to ensure that the entries support your intended basis for non-residency. In addition you need advice on the factors that HMRC will use to determine themselves if you really are or not UK resident. Day counting is no longer sufficient.
The P85(s) form is a shorter version of the form but the same care is needed. This shorter version is only applicable to people who are not UK citizens and who came to work in the UK for a work assignment.