We all know the changelist went from 816988 to 889555 but it's a bit harder to know what changed in fact.
I did a full comparison of the XWLS8 (FOP) and XWLSD (NEE, multi-csc) packages and here's a summary of what was touched / changed.
Kernel
- gannet (wifi related, I think - perhaps calls over wifi?)
- wifi driver updated
- exfat driver updated
- configuration of the RIL service on the init script
I don't know any more details, in particular what were the specific changes, whether the battery is affected, etc.
Libs
- libandroid_runtime
- libpolarisviewer4 (newer app version)
- libril and libsec-ril
- libstagefright* (audio encoding and decoding)
- libweb*
Framework
Already mentioned before by Goldieking; there are only very minor differences in:
- Sim unlock screen
- Wifi Ganlite (gannet, I guess)
- handling of Indian and Thai selection boundaries in text editors
- web viewer
- pause html5 audio when focus changes
- package manager (handling hidden packages moving to a different location)
Apps
- Additional apps: Google Magazines and Google Music
- Live wallpapers (?)
- Csc modem settings
- hidden dial codes *#07# (India only, shows country certificate info) and *#7547# in factory tests
- FM Radio - changes in the rename dialog
- KiesWifi
- Logs - permissions required for accessing the contents
- Newer version of Polaris Viewer
- Newer version of Samsung Apps
- Samsung keyboard (changes include some Turkish specifics)
- Samsung calendar
- Contacts (minimum chars for contacts match?)
- Launcher - handling of certain shortcut types (?)
- MediaProvider - handling of data upgrade (might it be related with mediascanner problems when not coming from a full wipe???)
- Samsung Mini Diary - some new features (?), a couple of screens changed
- sms/mms - small change in spam numbers list
- (Samsung) Phone - some changes related with Ganlite
- (Samsung) Settings - access points (security options, I think), development options (enable traces), lock screen wallpaper, proxy selector
- talkback option in setup wizard
(don't interpret all this as new features or big changes in all of them!)
As for the CSC part, there are settings for provider-specific APNs depending on the chosen csc, but since these also appear to exist on the global /system/etc/apns-conf.xml file for all the providers worldwide (plus they can be manually changed in the settings), it doesn't seem to be relevant which CSC is applied.
BTW, there was also a small change on apns-conf.xml for Vodafone Spain.