What is LTM SplitSession profile and how it works?

I can't seem to find any documentation clarifying how SplitSession profile actually works. I'm analyzing objects created by SSLO and so far my understanding goes as follow:

1. APM assigns LTM Connector profile to request using per-request policy
2. Bunch of connector profiles are grouped together into Service using Service profile so that down status of any VS defined for one service makes whole service "down" so that it can be eg. bypassed.
3. Virtual servers defined as connectors use split session profiles which do... what exactly?

I noticed that there are some other, interesting types of services that are unused by SSLO - for example Captive Portal. How could I use that? Would it be possible for example to put some VS with APM auth portal as one of services used in chain? Or is it meant for different use case?