EXIF datatypes interpretation

Question

I am looking into the EXIF format to write a parser. The tags are present here. http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/EXIF.html#LightSource

1)Some of the fields have 'undef' as the datatype. How would you use such a field? What type do we assume it to be?

2) Some datatypes are like this int16u[2]!. The exclamation means unsafe(when the cursor moves over it). But, what does that mean?

3) Another datatype is N. I don't understand what that means.

Answer 1

I see that the mentioned document has been revised at 13-Oct-2012, which is after you have asked your question. So I suggest to check again - maybe it has less "holes" now.