The Commons' vote on Friday will not quite be a third "meaningful vote" on Theresa May's Brexit deal, as required by the European Union (Withdrawal) Act 2018. UK politics nerds will know very well that Section 13 requires both the Withdrawal Agreement and the Political Declaration to be approved in order for the treaty to be ratified.
Rather, it will be on the Withdrawal Agreement only -- and not the Political Declaration,
What that means: The government is trying to square a circle. It needed to fulfil the Speaker's demand that another vote on the Brexit deal needed to be substantially different from before. As the leader of the House, Andrea Leadsom, told MPs, it also needs to get approval for the Withdrawal Agreement qualify for the extension to the Brexit process that was granted last week by the EU.
If the Withdrawal Agreement passes tomorrow, the Brexit date will automatically be pushed back to May 22 -- but MPs will still have time to debate the future relationship with the EU and approve that relationship by voting on the Political Declaration at a later time.