Bundle creator OS Type code ??? in Xcode 4
It's not really used anymore. It's a holdover from the Classic Mac OS days when it was a datapoint used mainly to determine what apps could create or edit or read file types. Back then you had to register them with Apple. I don't think they even provide that service any more. OS X and some apps might still use it in rare cases under the hood but its very legacy. As you can imagine, the severely limited possible permutations made it untenable long term.
UTIs are the preferred approach now. The system then uses a combination of these and file extensions and magic numbers. Though on ios that may not be true as much as OSX.
It is just the four letter code to identify the bunder creator... for example for apple it is APPL... It can be ???? or if your app name is "myApp" you can give "MYAP"... If you have many apps you can give first 2 characters with your app name and next 2 characters with company name...
For example apple uses CF for core foundation, AV etc