If the government assesses the brake has been used appropriately it then begins a process with the EU.
The Joint Committee - the UK-EU body responsible for overseeing the Brexit deal - is required to discuss the rule in question.
Once those discussions are concluded the UK government can either send it back to the assembly for a cross-community vote known as an applicability motion or decide the rule should not apply in Northern Ireland.
At that stage the government could still avoid a Stormont vote if it assesses there are "exceptional circumstances" including an assessment that the rule would not create a new regulatory border between Great Britain and Northern Ireland.
If the UK ultimately decides not to adopt the rule, the EU can take "appropriate remedial measures", which could include measures to address the fact that NI goods may no long fully comply with EU law.
In March, Stormont used an applicability motion to block a new EU rule on the protection of geographical indications (GI) for craft and industrial products.
At that time the government said it had noted the assembly vote and in October said it "continues to evaluate the implications."