Assuming that the deal is rejected, the margin of defeat is important.
Should May lose 30, 40, or even 50 votes, then she could stand up in Parliament and say that she will return to Brussels to try to optimize the deal.
If modest concessions were agreed with the EU, this could be enough to persuade some UK legislators to join forces in May in a second parliamentary vote. The EU has shown little interest in resuming talks. European Commission President Jean-Claude Juncker has warned the UK that May's Brexit deal is "the only possible deal".
If May loses heavily, chaos will likely be back in British politics.
The range of results runs from a leadership challenge, a May resignation, a parliamentary election in the UK, a second referendum on EU membership, a temporary halt to the British withdrawal, or a combination of all the above factors.