In some countries - including the UK - APD (a government tax) varies by class of travel (and also route). [See the HMRC website for UK details, you want notice 550 I think, www.hmrc.gov.uk]

As such, how it will vary will vary by departure location.

Note also, that the rules for application and criteria for classification when someone transits, connects or stops over in a country will also vary country to country.