One major difference between the new bill and the one that didn't get passed previously is that the new post-EU reality for the U.K. will not include a "backstop" at the border between Ireland and Northern Ireland. The U.K. includes Northern Ireland. Because Ireland and the U.K. have both been part of the EU, the 310-mile-long border between Ireland and Northern Ireland has been a "soft" one, meaning that no physical inspections of people and goods or restrictions on entry, such as passport control, have been necessary.

The new bill provides for a modified customs union between Ireland and the U.K. along that border. U.K. customs rules will govern any goods that go from non-EU countries into Northern Ireland and stay there. EU customs rules will govern those goods if they are intended to go on to Ireland.

In its debate on the bill that the Commons passed, the House of Lords voted 270–229 to amend the bill with a requirement that the U.K. government give citizens of the EU who live in the U.K. physical proof of their right to remain after the split. Proof can be granted online but does not result in any hard copy of the status and so anyone who wishes to check such status must go online. The concern is that the lack of a piece of paper proving citizenship status could be harmful to many people.

Under the U.K. constitutional system, any amendments to a bill go back to the House of Commons for consideration and can be overturned by the lower house. Effectively, the amendments make an even tighter timeline for the delivery of a deal to the EU by the mandated January 31 deadline.