Some goods coming into Northern Ireland from the rest of the UK would be subject to new checks and controls if the Brexit backstop is implemented.
The details are contained in a draft withdrawal agreement between the UK and EU.
The backstop would mean Northern Ireland would have to stay aligned to some rules of the EU single market.
That means that goods coming into Northern Ireland would need to be checked to see if they meet EU rules.
As the ultimate interpreter of those rules, the European Court of Justice would have a continuing role in Northern Ireland.
Speaking on Thursday, the prime minister said if the backstop was used the UK government could make a unilateral commitment not to have regulatory divergence with Northern Ireland.
She said that would "reduce the necessity" for new checks and controls.
How would goods be checked?
Guidance published by the European Commission (EC) says: "There would be a need for some compliance checks with EU standards, consistent with risk, to protect consumers, economic traders and businesses in the single market.
"The EU and the UK have agreed to carry out these checks in the least intrusive way possible."
Where will these checks happen?
For industrial goods based on risk assessment, checks could take place "in the market" or at traders' premises.
Such checks would always be carried out by UK authorities.
For agricultural products, existing checks at ports and airports would "be increased in scale in order to protect the EU's single market, its consumers and animal health".
Live animals coming into Northern Ireland from Great Britain are already subject to checks and controls.
Food products too?
Yes. It seems likely that those controls would have to be extended to some food products.
What does the business community think?
Aodhán Connolly, who represents the large supermarkets in Northern Ireland, said the deal "provides solutions to customs, tariffs, an innovative solution on VAT."
But Mr Connolly, who is director of the Northern Ireland Retail Consortium added: "There certainly is some work to be done by both the Westminster government and the EU on the checks and administration for goods coming in to Northern Ireland from GB.
"That will need to be light touch and be mitigated in further agreements, hopefully to an absolute bare minimum.
"However, this agreement must pass through Parliament. We have always said the backstop is better than no deal."
The deal means that goods going in the other direction, from Northern Ireland into Great Britain, would not be subject to new controls.
There's also the issue of premises certification. This is important in pharma & agrifood. Certification of NI sites by UK authorities will remain valid. My reading of it is that the EU will effectively trust UK authorities to certify EU standards in NI.— JPCampbellBiz (@JP_Biz) November 15, 2018
The agreement states that "nothing in this protocol prevents the United Kingdom from ensuring unfettered market access for goods moving from Northern Ireland to the rest of the United Kingdom's internal market."
Stephen Kelly, chief executive of Manufacturing NI, said the EU had made important concessions which would help NI businesses if the backstop is ever implemented.
"The fears and risks flagged by business seem, so far, to have been addressed," he said.
"The issues of VAT remains an HMRC responsibility and there will be no quotas, origin rules nor tariffs between Britain and Northern Ireland," he added.
What about labelling?
Under the backstop there would be new labelling requirements for Northern Ireland produced goods.
Northern Ireland products being sold in Great Britain could be labelled as being from the UK or Northern Ireland.
But Northern Ireland made goods being sold in Northern Ireland or the EU could only be labelled as being from Northern Ireland, not the UK.
The actual labelling mark for Northern Ireland products will be UK (NI).
The agreement also explains how, if the backstop was implemented, Northern Ireland produced goods would be certified and approved.
How will products be certified?
Products being made only for the UK market would not need to go through an EU approval process.
They would follow the approvals and certifications processes they do today, through the same UK bodies.
Businesses selling products to both the EU & UK would need an EU approval but that approval will also be valid in the UK.
That means NI businesses selling in both the EU and the rest of the UK will not have to go through two separate approvals processes.
How will it affect business support?
The backstop would also mean the EU having a continuing role in deciding what sort of help businesses can get from government - what is known as state aid.
For aid granted by the UK that affects trade between Northern Ireland and the EU, the European Commission (EC) will continue to enforce the aid rules directly.
The European Court of Justice will also continue to have a role.
However, for aid granted by the UK affecting trade between the rest of the UK and the EU, the enforcement will be done by a UK enforcement authority, likely to be the Competition and Markets Authority.
UK courts will supervise the UK enforcement authority, but the EC will have legal standing before UK courts and the right to intervene in cases.
How might all this come to an end?
One major question around the backstop is how it can be brought to an end.
The Irish government's view has always been simply that it will last unless and until something else comes along which which means it is no longer necessary.
The agreement lays out in more detail a process for reviewing or ending the backstop.
If either the EU, or more likely, the UK thinks its no longer necessary they would have to notify the other side about why they think it should end.
Within six months of that notification there would have to be a meeting of ministers - that meeting could also seek an opinion from the Northern Ireland Executive.
However the ultimate decision would have to be a joint one between the EU and the UK.
One side could not choose to end the backstop on their own.