Will MPs be given a vote?
The High Court has already ruled that they must, but the government argues that it can be achieved by royal prerogative instead. Most people, including the government itself, believe the Supreme Court will agree with the earlier ruling of the High Court and have already prepared their response with this in mind.
What will Parliament vote on?
The best-case scenario from government's perspective, if they lose today's case, is that the court will order ministers to pass a simple one clause bill authorising Article 50. This would be a simple yes/no clause which would not be open for amendment. However, there is a very good chance that the court will suggest that an additional clause could be necessary, which would allow MPs and the Lords to amend the bill. This would open up the possibility of Parliament seeking to attach conditions to their support for Article 50. This would be fascinating because although Labour has said they will vote for Article 50 come what may, it is possible that a majority of MPs will be persuaded to attach some form of condition to that support.
Will it be a unanimous decision?
There is a possibility that the court's final ruling is not unanimous. While this would not change the final outcome it could give campaigners on either side backing for their attempt to influence the subsequent parliamentary process.
Will UK nations get a veto?
The absolute worst-case scenario for the government would be if the Supreme Court decide that the Sewel Convention applies. Under this convention Westminster does not "normally legislate with regard to devolved matters in Scotland without the consent of the Scottish Parliament." Given the impact that withdrawing from the EU would have, the Scottish government argue that they must be consulted. Most legal experts believe the Supreme Court are unlikely to support this, but it is by no means impossible. Any such ruling would put a pack of dynamite on Theresa May's plans to trigger Article 50 by March.
Could it all end up in European courts?
There is a small chance that if the Supreme Court does decide that the ruling must go to the devolved administrations, then the whole thing could ultimately end up being referred to the European Court of Justice. This seems very unlikely, but if it does occur it would be a disastrous result from the government's perspective.
Source: Adam Bienkov - Business Insider