NEXT_PROTOCOL
NEXT_PROTOCOL is a method that can be used to determine the protocol at the next layer in a stack. In the case of IP for example, this would determine if the next layer is TCP, UDP, or one of many other less common possibilities. The method returns a value that is looked up in an personality file to retrieve the standard FTE id for the next protocol. (Now you know one reason why every protocol is required to have a unique id.)
IP has a field, "protocol", that has a unique value and determines the next protocol to use, so we call a method that picks that field out.