The new enum in Objective-C

In the latest tools, a new kind of enums are now allowed:

typedef enum CarType : NSUInteger {
  FourDoorCarType,
  TwoDoorCarType
} CarType;

My question comes in parts:

1. Why should I use this instead of the old way?

2. Why does CarType appear twice? I tried skipping the first CarType and just leaving the first line as "typedef enum : NSUInteger {", and it seems to work fine. What are the drawbacks, if any?

3. Can some types other than NSUInteger be used?