MMS and Restricting background data Using GT-I9300, JB 4.1.2.

I can't seem to find details on how "Restricting background data" works (with Mobile Data enabled) exactly.

What is considered "background" data? 
E.g., if I'm sending an MMS and the phone goes to sleep, will the MMS be sent? If I'm sending an MMS and press the Home button, will the MMS be sent? 
Do I need to have the Messaging app open all the time for it to be sent? Is it okay, if I'm looking at the settings of the said app?

I've asked questions pertaining to MMS, but I'm also wondering about the general case.
Background data is anything that involves mobile data or Wi-Fi; it will use whichever is enabled (I think Wi-Fi has higher preference). This includes autosync, background polling, notifications, and other similar operations that will access mobile data.

Assuming you don't have any task / data killer that runs in the background when your phone sleeps, the MMS will be sent You do not need to have the Messaging app open. However, if the sending fails (after a certain number of retries), it will fail and you will have to resend it manually.

Update #1: Autosync depends on background data. If background data is disabled, autosync is also disabled. Autosync is used by accounts defined in settings (In S4, System Settings &gt; Accounts). 

BUT it's also possible to have background data enabled, and autosync disabled. In this case, apps could poll and show notifications (e.g. Twitter).