When should I use a Custom Field vs an Item?
To decide whether to track a piece of information in a custom field (on the Custom page) or in an Item (on the Assigned Items page) of a member or group record, you have to ask: "Can a record be assigned this kind of data more than once?"
If the answer is yes, then you don't want to use a custom field, because a custom field only holds one entry. Thus, the second time a member is assigned this "thing", you would have to use another field for it, when it's really just an extension of the first.
For example, say you need to track the member's Driver License number. Typically, a person will only have one of those, so that's a good candidate for a custom field (or possibly for the Member ID).
But if you want to track something like, "Donated Auction Items", "Offices Held", or "Degrees Earned", a system of Categories and Items is much better.
Take the example of Degrees Earned. If you create a category called "Degrees Earned", you can create items under it for "PhD", "Doctorate", "Masters", "BS", "BA", "MBA", etc.
Then, if a person earns her bachelor's, you can go to the Items tab on her record and create a record for "BS" and even set the date and a comment (maybe the university name). Then when she gets her Masters in business administration, you can go back to her record, go to the items tab, and enter another record for "MBA" with the date and comment.
Since this person is going on to law school, you know you'll be back to add another item. If you had used Custom fields it would have been much more difficult.
Further, if you want to run a report and show everyone who has earned a masters degree, you can just use criteria like:
Assigned Item = "MBA"