A question about flashing new roms

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http://forum.xda-developers.com – Hello im a bit confused about a couple of things so i hope anyone could help me : 1- S-off vs Unlocking bootloader : are they equivalent ? i mean if i dont want to s-off my phone can i flash ANY custom rom just by unlocking bootloader and installing custom recovery ? (custom rom such as "Android revolution HD" ) 2- I want to flash a "stock rom" that has a different cid than my phones, i have the ruu and i managed to get the rom.zip , how could i flash through sd card ? do i need s-off ? or is unlocking the bootloader enough ? or do i need to do anything else ? Hope y (HowTos)