This autumn, two words will matter more than any other when it comes to Brexit: “sufficient progress.”

It is the great test Brexit negotiators will face — barring something unexpected — ahead of the European Council’s summit in October. As set out in the EU27’s negotiating guidelines, the European Council (minus Britain, of course) will decide whether talks over the summer on so-called exit issues — notably citizens’ rights, the Northern Irish border and the Brexit bill — have progressed far enough. Only then will they permit negotiators to move on to the next stage, and the bit the U.K. really wants to talk about — the future relationship with the EU and trade.

EU chief negotiator Michel Barnier intends to update the Council either at or around the time of its next summit on October 19 and 20.

“What’s sufficient? How long is a piece of string?” asked the U.K.’s former Brexit Minister David Jones. “I think it’s going to be more of a political decision than anything else.”

European Council officials acknowledge that the meaning of the phrase is “not specified.” That allows for a great deal of flexibility when the EU27 leaders meet. Ultimately, it will be a subjective judgment rather than a decision made against a checklist of negotiation goals. The thinking of each leader is likely to be steered by the most powerful Council members — Angela Merkel (assuming the German chancellor wins the upcoming September 24 election as expected) and France’s Emmanuel Macron.

According to U.K. officials, “sufficient progress” won’t necessitate putting an actual figure on Britain’s financial liabilities. On Ireland, it’s likely nothing will be settled until the end of the Brexit process. Meanwhile on citizens’ rights, the role of the European Parliament could become significant, introducing a second political dimension that the U.K. will need to be aware of.

Brexit bill bust-up

Success or otherwise in the “sufficient progress” test is likely to hinge on the key issue of the U.K.’s financial obligations — the requirement, as the EU sees it, to meet commitments already made toward the EU’s budget up to 2020.

The EU will at least expect the U.K. to set out its proposal for calculating what its obligations are, if not a final figure. Thus far, the U.K. has refused to do so, preferring to critique the proposals put forward by the EU negotiating team. During the third round of Brexit negotiations, it presented a legal analysis of the EU proposals which rejected the entire basis for the bloc's calculations. It favors of a more à la carte approach to the budget, with the U.K. paying for those parts (so far unspecified) that it wants to be part of.

At the press conference rounding off the talks, David Davis, the U.K.'s Brexit secretary said he had a duty to "interrogate" the EU's position on the budget. “It’s fair to say, across the piece we have a very different legal stance,” he said.

“The key issue for ‘sufficient progress’ will be money,” said Charles Grant, director of the London-based Centre for European Reform think tank. “The EU and the U.K. will have to agree on a methodology for calculating the amount that the U.K. must pay.” Such a methodology would have to be “detailed enough to reassure the EU … but vague enough so that journalists cannot easily work out a direct figure,” Grant added. EU estimates of this figure range from €65 billion net to €100 billion.

One reason for the vagueness is it would be politically difficult for Prime Minister Theresa May to present a bill to the British public in October before she has any perks, such as trade concessions, to show for it.

Having to settle on a figure to reach the next stage of talks would, in the words of one individual familiar with the British position, be seen by Brexit-supporting British newspapers as “paying £50 billion for a conversation.”

But U.K. officials are clear the Brits will pay something. Downing Street says it does not recognize reports in the British press that the opening offer would be £36 billion, staggered over three years of a transition period. Some Brexit-supporting Conservative MPs said even this was too much, creating a potential political headache for May. But even the Brexiteers are coming around to the idea that some money will have to change hands.

British Foreign Secretary Boris Johnson, who was closely associated with a discredited Vote Leave campaign pledge to repatriate £350 million a week from the EU budget, has subsequently conceded that the U.K. will have to pay something on exit.

But he may be unwilling to defend a big payout, Grant said. “It would make him look silly, after what he said in the referendum campaign … he might choose to resign rather than defend the deal. But the U.K. will have to pay ... or the EU will simply refuse to discuss the future relationship.”

Citizens and their Parliament

On the question of the rights of EU citizens in the U.K. and U.K. citizens in the EU, the European Council will be under pressure from the European Parliament, which intends to formally indicate, prior to the Council’s decision, whether its Brexit steering group, led by former Belgian Prime Minister Guy Verhofstadt, believes “sufficient progress” has been reached.

In a statement issued July 25 following a meeting with Barnier, the group set out a series of red lines, including safeguarding family reunion rights that the U.K. wants to water down and protecting rights to vote in local elections, which the EU wants to water down. It also sets out its opposition to a U.K. plan to carry out systematic criminal records checks on every current EU resident who applies to stay in the U.K. after Brexit and advocates a “full role” for the European Court of Justice in safeguarding the rights of citizens on both sides after Brexit.

While the European Parliament position will only be advisory for the Council, one EU official speaking on condition of anonymity said the Council would be wise to heed it. “Parliament has a vote on the Brexit deal at the end of the process, so its wishes have to be taken into account when deciding on ‘sufficient progress,’” the individual said.

Ireland

The Council is likely to be lenient in its assessment of “sufficient progress” with regards to talks on the border between Northern Ireland and the Republic of Ireland. While officially classed as an “exit” issue rather than a “future relationship” one, there was early agreement in the Brexit talks that the political sensitivity and the importance of preserving the peace process made this set of issues a special case. It has been allocated privileged status within the talks; a “dialogue” led by two of the most senior officials from either side — May’s Brexit sherpa Oliver Robbins for the U.K. and Barnier’s deputy Sabine Weyand for the EU.

One official familiar with the progress of talks said there was an understanding on both sides that the matter would not be resolved until it was clearer what the terms of the U.K.’s trading relationship with the EU would be — so not until the end of the Brexit talks process.