Growstuff and Wouldbegran, thank you for your replies.
The originally discussed backstop applied to NI only so there would be no hard border in Ireland, so no trade tattoos, extra costs for farmers, etc. NI would effectively stay in the Customs Union, whilst the rest of the UK would leave it. Goods entering &leaving the island of Ireland (N&S) would be checked at ports only, not at the border, and there would be no tariffs on the island . At the moment there are no checks, as agreed in the Good Friday Agreement between UK and Ireland N&S. If they reverted to that, the checks would be at the ports or, as the DUP say, there would be an economic( not political border). The original was changed so the whole UK including NI would be the same, to get the DUP to support the Cons Party.