from declaration on application for registration

Thanx

If you are sending mail by DX please make sure that the persons name, company name, DX number and exchange are shown clearly on the front of the envelope.
DX information:
Name PKF
DX number 15570
Exchange Lincoln 4

И что такое DX?

DX is the UK's leading independent provider of early morning, next day business mail services and currently delivers over 360 million items per year throughout the UK.
...
DX is the only UK-wide provide of Document Exchange services – a unique private mail network that provides delivery of time-critical mail to around 27,000 business professionals drawn from sectors such as Banking, Law, Architecture, Estate Agency, Government and Accountancy

The 'DX number' is a number, assigned by the International Imaging Industry Association, that uniquely identifies the type and manufacturer of a film emulsion. A list of assigned DX numbers for APS and 135 film is issued annually.
See also, but not to be confused with, DX encoding

http://wiki-shorts.freestat.pl/15-1318-DX_number.html

Я даже расшифровки DX не нашла! Вот и спрашиваю!

The 'DX number' is a number, assigned by the International
Imaging Industry Association, that uniquely identifies the type and ...

IHTM02141 - Communications: British Document Exchange: what is British Document Exchange?
British Document Exchange (otherwise called ‘Britdoc’ or ‘DX’) is an alternative to the Royal Mail for sending business letters to firms. It is a licensed private postal system whose membership includes many solicitors and accountants and some Government departments.
The service offers a considerable saving over normal postal costs, and
undertakes to deliver all DX correspondence by the following day
All members of Britdoc are required to display their
DX number, and
exchange location
on their headed notepaper