Item Type Metadata and File Metadata?

What is the different between Item Type Metadata and File Metadata (in Edit File Metadata) it necessary to fill up the file metadata? because there are dc fields listed inside too (as well as Format (Legacy) Metadata...) could duplicate the input job...

Sometimes an item contains one file, but an item may contain multiple files. In some cases, users might want to store additional DC data that is specific for each file.

It is not necessary to add file metadata, but the option is available for those who may want it.

Sheila, thanks for the is good feature and option..I wanted to fill up but got not enough staff and time.. :)