...how does the ability to copy info from one field to another help me?
I don't know. You were asking how to use custom fields. I was just suggesting that as one possible way to get information into them without having to add it manually for each file or group of files.
Basically, in MJ8, the three custom fields are just simple data fields. You enter information into them the same way that you do with any of the other fields like artist, album, etc. AFAIK, there is no way to "associate" them with any file properties such as the filename. As I said before, they are there for you to use for information that may not fit into any of the default fields. For example, I renamed one of the custom fields to
Source. Then as I added new files to the library I specified whether the file came from a cd I own, a cd I borrowed, download, from a cd-r, etc. Once you enter info into a custom field, you can then add it as a sorting column in your track view and/or use it to build view schemes, playlists or smartlists.
In MC9, in addition to the three default custom fields, you can also create your own custom fields and specify what type of info they hold, such as string, integer, semicolon delimited, etc.
Rob