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Bis street numbers in Belgium may be of alpha or numeric nature. They are not to be confused with extension designations (box numbers). An alpha bis street number will immediately follow the number, for example 21C. A numeric bis number will be separated from the street number by a slash (‘/’), for example 21/3.

Extension designation (box numbers), if present, shall appear in the delivery point location line, preceded by the literal ‘box’ (‘bus’ in NL, ‘bte’ in FR). The literals ‘b’, ‘Bt’, ‘#’, ‘‐’, ‘/’, … are not allowed as separators between the street number element and the box number extension designator element. The element building/construction, if present, appears on the line above the delivery point location line.
https://www.bpost.be/sites/default/files/pagina/Documentatio...

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Note added at 14 hrs (2019-05-27 10:28:28 GMT)
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From the same document:

Address Element: Box Number
Title: Extension Designation
Code: 40.28.0.0.0
Comment: If not null, and must be preceded in representation by literal ‘bus’ or ‘bte’, or ‘box’

Example:

Private address of a person living in a block of flats
Formatted address
Dhr Paul Janssens
Kortijkstraat 37 bus 1
9800 Deinze

bus 1 >> box 1

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Note added at 14 hrs (2019-05-27 10:30:39 GMT)
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Company address with spatial information
Formatted address

Dhr Paul Janssens
Accounting
Acme NV
Building West ‐ Verdieping 4
Bloemendalelaan 62/3 bus 47
9990 Maldegem

bus 47 >> box 47

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Note added at 14 hrs (2019-05-27 10:44:03 GMT)
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Use the terms "bte, bus, box, boîte" in front of the box number

Rue de la Senne 32 bte 20
Zennestraat 32 bus 20
Rue de la Senne 32 box 32
Rue de la Senne 32 boîte 20
http://bpost.be/site/en/address/how-compose-address