Unable to find information in the product docs about "split choices" other than a single reference in Understanding Split Campaigns.
Closest reference relates to split tests, A/B testing.
I'm hoping there's more......
What are you trying to accomplish?
I don't think there is a lot of documentation on split choice because A/B tests will probably serve most purposes.
But if you wanted to split an audience for example, you could add choices in your flow step. So for Send Email flow step, you can add a choice that says If random sample is 50% then send email A, and the default choice would be send email B.
I use choices sometimes to send emails based on some criteria, it is an easy way to reduce the number of email smart campaigns you use. So for an event you could use one smart campaign to send a second invitation to those who have not registered (choice B being status is invited) and then the default choice would be send a reminder to attend (in this case you would use another choice A that says status is registered) and default choice would be "do nothing".