Now don't nail me on it I'm not quite sure and didn't recherche, but isn't it the "old fashioned" UK word for p.o.box?

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Note added at 2003-08-28 05:05:57 (GMT)
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Okay got it! P.O. Box is nonsense. Mail stops are used in large institutions like government buildings, university campuses, and such. Damn near anything that doesn\'t have street names but more than one building. Usually over here things aren\'t all that big (several buildings, one institution) The closest thing I found is a \"Postverteiler\". I still couldn\'t find a literal translation, but at least you know what it is. When you enter the term in google the first 50 or 60 entrys are indeed about university campuses.

When you follow Johanna\'s USPS link you\'ll find a nice example about the patent comissioners office

Mail Stop 12
Contributions to the Examiner Education Program.

Mail Stop 313(c)
Petitions under 37 CFR 1.313(c) to withdraw a patent application from issue after payment of the issue fee and any papers associated with the petition, including papers necessary for a continuing application or a request for continued examination (RCE).

Mail Stop AF
Amendments and other responses after final rejection, other than an appeal brief.

Mail Stop Appeal Brief
Patents For appeal briefs under 37 CFR 1.192 or reply briefs under 37 CFR 1.193(a).

Mail Stop Application Number
For fee and petitions under 37 CFR 1.182 to obtain Mail Stop Designation received and/or application number for patent applications prior to the Office\'s standard notification (return post card or the official \"Filing Receipt,\" \"Notice to File Missing Parts,\" or \"Notice of Incomplete Application\").

Mail Stop Comments
Patents Public comments regarding patent related regulations and procedures.

Mail Stop Conversion
Requests under 37 CFR 1.53(c)(2) to convert a nonprovisional application to a provisional application and requests under 37 CFR 1.53(c)(3) to convert a provisional application to a nonprovisional application.

and so on and so forth. It is basically an address within a postal mailing address, used by internal mail distributors.
Good luck with it.

In a large organization, a lot of mail is only moved internally, from department to department, even different states, without really involving outside postal services. Your internal address is usually included on your business cards. We used to call it the internal mail code.