When using Azure AppFabric Cache as a Session Stat

Question

I have AppFabric Cache configured as the Session State Provider for an ASP.NET MVC application. Not all pages access session state (in fact only a few ajax requests do, the main pages commonly accessed by the site users don't, in order to make them cacheable) Give the hourly transaction limit on AppFabric cache, what causes the Session Provider to execute a transaction with the Cache ? (See @knightpfhor answer on Estimating simultaneous Azure Appfabric Cache Connections for the source of this query)

Is it

1. any resource (inc images / css)
2. any request/page executing code
3. any request/page with an explicit Session reference
4. something else?