There are many different types of tax investigation that are undertaken on a civil basis (as opposed to a criminal basis).
HMRC can enquire into a taxpayer’s past tax compliance under the authority of different tax legislation. The legislation used can depend upon whether the taxpayer is an individual, partnership or limited company.
HMRC will also seek to abide by various Codes of Practice (which are not law).
A tax enquiry can refer to a tax investigation, a tax dispute or simply a challenge being made by HMRC.
If in doubt, clarification should be sought directly from HMRC. HMRC will clarify the relevant legislation and Code of practice being used into which taxpayer(s) for which tax year(s).