See: http://www.postalreporter.com/automation/pars_glosary.htm

This is not my field, but I came up with the same link as you did that uses "sorting assignment surface." It seemed to make sense to me when I read the English and Japanese versions. I thought "assignment" can also be translated as "designated."

- 区分 is used as in分類する terms of mail as in as in sort (out) mail.
- 指定　– assigned or designated. Designation means “identifying word or words by which someone or something is called and classified or distinguished from others.” I thought this could apply in sorting mail.
- 面 is defined as aspect, face, mask, surface, plane, side, facet, etc. I am going with “surface."

The below website on "sorting mail" talks about the "sorting surface" which I believe is the surface that is "read" by the sorting machine for the sorting operation.

http://www.freepatentsonline.com/4601396.html
This invention concerns a method for sorting flat and indexed articles as well as a device for carrying out said method, both of which are particularly applicable to the sorting of mail.
A first part of conveyor 4 is determined by a belt 28, termed the read belt, going past read head 5 and by another, magazine 1 extraction belt 29. A second part of conveyor 4 is determined by the same belt 28 and by a buffer routing belt 30. A third part of conveyor 4 is determined by said belt 28 and by the “sorting surface” 31 of a diverting vane 9.

Here is another good "mail sorting machine" website:

http://www.freepatentsonline.com/5431288.html
A mail sorting apparatus includes a video signal conversion section for photoelectrically converting and quantizing characters, codes, and the like on a mail item into a video signal, a blank area detection section for detecting a blank area where characters, codes, and the like are not described on the basis of the video signal output from the video signal conversion section, a printing area designation section for designating a bar code printing area in the blank area detected by the blank area detection section, and a bar code printing section for printing a bar code corresponding to an address of the mail item on the printing area designated by the printing area designation section. Further, the printing area designation section designates the bar code printing area from a blank area obtained by excluding a window area detected by a window position detection section from the blank area detected by the blank area detection section.

区分 is a fairly generic term that I have seen used to indicate a group of arbitrarily defined categories/classes/selections in user interfaces. For example, where I have seen it used personally is on software user interfaces where the user is to select between a defined list of options.

For example, there is a screen with a form interface where the user has to select from a group of colors for a product they can customize. The group of colors itself can be referred to as カラー区分 or 色区分 literally, color categories/selections. In other words, 区分 is used to directly express a category or a group of selection in a category in Japanese whereas in English it is implicit (ie. in a selection of a list of colors, the title we would use is "Colors" and not "Color Categories" and the like).

A constrained example in the case of a mail sorting machine I can think is a sample 区分 that could be specifying the type of prefecture the target address is located in (eg. allowing the user to select from 都, 道, 府, and 県).

According to WwWJDIC, 指定 means to indicate something or in this case, to select or set something. That something I am guessing are machine settings of some sort.

According to WWWJDIC, 面 literally means face and surface sure but that doesn't really help us in this context. At first, I thought that 区分指定面 should actually be 区分指定画面 so that it would mean an interface screen where the user can adjust various settings by choosing between a bunch of predefined selections. But then you said that this phrase was used in the context of a physical machine so my guess is that it refers to a physical panel where the user can adjust various settings hence "category selection panel" or "settings panel". "Settings panel" seems very generic compared to 区分指定面 so I would look into what sort of settings the panel allows the user to adjust and then add the appropriate adjective to describe what kind of settings panel it is. This will of course detract from the literal meaning of the original Japanese but will make the English version sound more natural and informative, at least in my humble opinion.

Of course it could be a variety of settings that exist only in Japanese specific protocol etc. where there is no sufficient English translation and then you also have to fit your long English string into the constrained space that contained the original Japanese etc. etc. Good times.