I am planning on visiting Japan from the UK and was looking at the customs forms which mentioned "unaccompanied baggage" or luggage.
It counts as "accompanied". You pass through customs after you've collected your bag, so at that time you will be accompanying the bag.
There is often an airport staff member walking along the Foreigners' passports queue checking forms, and in my experience they are friendly and happy to answer any questions you have.
For the Customs Form that you fill out on your flight, the question about "Unaccompanied Articles" refers to any personal effects you have sent by other means for use while in Japan.
For locals this refers to items mailed home or sent by other means, intended to keep folks from exceeding their import limits by sending some items by alternate means.
For tourists this refers to items, like golf clubs, bicycles or other bulky items you may have sent by a baggage service or post. Again so that they are aware of everything that you are bringing in.