SALZBURG, Austria (Reuters) - A “backstop” policy on Northern Ireland’s land border with the European Union is a key obstacle to Britain securing a deal with Brussels on an orderly exit from the bloc in March. This is what it is and why it matters:

FILE PHOTO: A Brexit sign is seen between Donegal in the Republic of Ireland and Londonderry in Northern Ireland at the border village of Muff, Ireland, February 1, 2018. REUTERS/Clodagh Kilcoyne

BOTH SIDES WANT TO AVOID A “HARD BORDER” - WHAT’S THAT?

In 1998, Britain and Ireland made the Good Friday agreement to end 30 years of violence in Northern Ireland over whether it should stay British or join the Irish republic to the south. That ended controls along the 500-km (300-mile) land border and set up many all-Ireland rules and institutions. Brexit would mean the north leaving the EU and so could mean checks along the frontier again and different trade rules that could inflame tensions once more.

SO THE SOLUTION IS...?

British Prime Minister Theresa May says a special customs and trade arrangement with the EU, to be negotiated during a 20-month status-quo transition period, will ensure seamless borders between the whole of the United Kingdom and EU -- so there will be no need for intrusive checks on goods at the Irish border.

THAT SOUNDS FAIR - SO WHAT’S THE CATCH?

Two catches really. EU negotiator Michel Barnier says May’s “Chequers” proposals for future customs and trade in goods give Britain unfair access to the EU single market. Second, the Irish government, backed by the rest of the EU, wants an insurance policy in case those future trade talks fail.

AND THAT INSURANCE POLICY IS WHAT?

Early this year, Barnier proposed that, if a better solution involving the whole UK is not found before the end of transition in 2020, Northern Ireland alone would effectively remain in the EU economic space for as long as a better solution was lacking. That would mean the province applying EU regulations and customs checks and hence monitoring trade with the British mainland.

BUT BRITAIN OBJECTS, RIGHT?

Yes. May and her vital allies in a pro-British Belfast party reject any border between two parts of the UK. Northern Irish unionists fear that would push the province toward a merger with the republic. Britain suggested applying the backstop customs deal to the mainland too. Brussels dismissed that as a dodge to get British goods into the single market by a back door. At the heart of arguments is not so much customs duties, though they do matter, but checks to make sure goods are produced in line with EU rules -- from which Britain aims to be free after Brexit.

COULD THEY JUST DO A BRUSSELS POLITICAL FUDGE?

There could be a temptation to leave some loose ends on Ireland to be sorted out in talks on the future EU-UK trade deal. But Dublin, backed by the EU, demands a legally watertight backstop that it could hold Britain to in international courts.

SO WHERE DO THINGS STAND NOW?

May says she is willing to agree to a backstop to assuage EU and Irish worries - but not to a border in the Irish Sea. Barnier is offering to rework the insurance policy to “de-dramatize” fears in London and Belfast, insisting it poses no threat to Britain’s constitutional integrity. There would be no border as such between the province and the mainland but checks, often by British officials, at factories, markets, on ships and in ports, on the status of goods moving between the two. He is also stressing that it may never be needed, assuming that the EU and Britain will indeed agree a close trade agreement by 2020.

AND WHAT HAPPENS NEXT?

May will appeal to EU leaders over dinner in Salzburg on Wednesday to understand what is unacceptable to Britain. The 27 will discuss that over lunch on Thursday and summit chair Donald Tusk will then brief May in Salzburg on those discussions.

May must survive criticism of her plans from hardline Brexit supporters at her Conservative Party conference early next month and then diplomats expect Barnier to unveil new draft terms. He hopes for a “moment of truth” at an Oct. 18 summit in Brussels, where leaders could agree the outline accord with May.

That would give negotiators a month to nail down a final treaty and accompanying political declaration of intent on a future trade deal at a special Brexit summit in mid-November.

The British and EU parliaments must ratify the treaty before Britain leaves on March 29, ushering in the status quo transition which ends, under current plans, on Dec. 31, 2020.