If I understand correctly there are two parts to the backstop in the European Union (Withdrawal Agreement) Bill.
First, if the future relationship has not been agreed before December 2020 then “backstop 1” is triggered whereby the UK (or is it just Northern Ireland?) remains inside the Single Market and Customs Union, with the commensurate regulatory alignment. This is timebound to “one or two years”..
Second, if the future relationship has not been agreed by December 2022 (ie. the end of the “one or two years”, then the UK remains inside a customs union with the EU in perpetuity, until such time as a solution to the Irish border is agreed.
Is my understanding correct?