basic idoc type

While creating a basic idoc type there is a radio button 'Extension' on the screen.

When i specify a basic idoc type name and click on the create button,it takes me to the create screen,which has some other options like 'Create new' 'Create as copy' and 'Create Successor'. My ques is that what is the difference between

a) Creating a basic idoc type using radio button 'Extension' on the we30 screen.

b) When I click on radio button basic type on the we30 screen and then in the next screen if i Click on 'Create Successor'.

Are both of them doing the same thing?