IEIM111010 | Exchange of Information: Outward requests: Overview
From HM Revenue & Customs · International Exchange of Information Manual
Exchange of information requests are a normal part of HMRC enquiry work. An outward request can be made at any stage of an enquiry providing:
There is a relevant legal gateway for the request (IEIM101200);
The information you are seeking is foreseeably relevant (IEIM101300); and
It is information the UK would be legally able to provide for a similar inward request from the other tax authority and so meets the standard of reciprocity (IEIM101500).
You must also consider whether you have exhausted all domestic avenues (IEIM112300).