The XiBs are the same thing as their XB counterparts as they are used now. They are supposed to replace them because the XB prefixes (mega, giga, etc) are wrong according to standard SI prefixes. i.e. A kilobyte (KB) means 1,024 bytes, but it's not supposed to because kilo means 1,000. So some organization changed the name to kibibyte (KiB) which means 1,024 bytes. I think this is because when you get into the higher numbers, there's a much bigger difference than 24 bytes. But there hasn't been enough of a push to change general usage.