The Sub-Tiers of PvE Raid gear (MoP)

I have noticed that there are 5 'Sub-Tiers' for the gear dropped in Raids:

1. Heroic Warforged - iLvl 572 (Max 588) [Upgrade: +6]
2. Heroic - iLvl 566 (Max 582) [Upgrade: +7]
3. Warforged - iLvl 559 (Max 575) [Upgrade: +6]
4. Normal - iLvl 553 (Max 569) [Upgrade: +13]
5. Flexible - iLvl 540 (Max 556) [Upgrade: +12]
6. LFR - iLvl 528 (Max 544)

I Understand that LFR drops from LFR Raids, Flexible from Flex Raids, Normal from Normal Raids and Heroic from Heroic Raids.

I have noticed that Warforged gives +6 item Level.

What I do not understand, is how to get the Warforged items? Do you have to manually do something to upgrade them? Are there special conditions under which you could loot these?

Also, can you get the Set Pieces (Head, Shoulders, Gloves, Legs, Chest) as a Warforged item?