Ext_csd is a configuration file ,which does petition in blank emmc in four types. boot1, boot2, rpmb and user_area. ext_csd chooses in which starting application of the petition the phone will be on during boot. But extcsd don't petitions the user_area. The preloader or .xml decides which file of the firmware under user_area will be written .
Pit is also same as ext csd. But pit allows emmc pertitioner along with user area to pertition firmware or write to be ready.
(NOTES BY MRINAL.M-REPAIR,GUWAHATI,ASSAM