(keitai-l) 504i

Hello all.

This might be a stupid question, but...

When people say about 504i handsets, do they mean that:

 1) There's a strict defined specification that rules what
 capabilities and features all the handsets must have to belong to
 504i group.

 2) It's just a common word to unite handset models of the same
 generation (manufactured after certain date).

In other words, what makes handset a 504i handset?

Thanks,
 Andrew