What is an OS2SD ROM? I was going to install this CM 12.1 ROM to my phone, but it is listed as OS2SD. What is OS2SD? I could not find anything about it on the internet. Also, the thread says this: 


  Notes for OS2SD:
  Before you start: I cannot stress enough cwm backups/restoring will not work with this OS2SD release! And data MUST be formated NOT Optional!
  1. You MUST Backup All data or it WILL be lost!
  2. You MUST Format /system, /data, /cache, and /sdcard0 in cwm/twrp.
  At this point you can format sdcard0 as ext4.
  3. Install the new CWM/TWRP the old one will not work from now on!
  If you didnt before now would also be a good time to format the internal storage which in the new recovery is now /data to ext4.
  4. Once formated first flash the OS2SD rom
  5. Then flash GAPPS and/or other packages.
  At this point you can restore data. Note the kernel is special so you cant use older ones so you can only restore things like /data backups and adb/titanium backups.
  6. Then reboot and enjoy the extra space!


Does this mean that my CWM backups won't work? And does OS2SD mean it will format and install to my SD card? 

Thanks!
Judging from the name OS2SD (Operating System TO SD) and from the line that reads OS2SD friendly TWRP recovery, handles partitioning of sd card for OS2SD in this XDA post, it seems that it's a particular flavor of Android that's made to be installed on an appropriately formatted external SD. Aside from that, I was not able to get more info.

Anyone is invited to improve this answer with more info.