What is the use of ExtraData in Setupp.ini?

I searched high and low for more info regarding the "ExtraData" portion of a setupp.ini but could not locate any articles that describe it properly..

Anyone here have/know what is the use of this "ExtraData" strings for??

I know that "PID" is modified to accept diff type of keys but surely there's something the "ExtraData" portion is use for..

ExtraData=786F687170637175716954806365EF

One thing that I know (from part of a post here) is that the sum of the string in hexbyte-size will produce the last byte of the string as a checksum.

Anymore that is of use for this string since every version of Windows comes with different ExtraData string?