Seems to me the distinction is between tipping and non-tipping.

I first thought it was between tipping tankers and powder tankers. However while many powder tankers are non-tipping - because they use compressed air to drive out the contents - powder tankers can also rely on tipping.

Notes:- KIP 48.3 Tipping powder tank,Single front end tipping ram. BPW Tri-Air Alcoa, ... Body Type: non tip. Product: POWDER. Capacity: 49cm. Compartments: ...
www.intank.co.uk/sales.html -

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Pneumatic dry powder tankers: – B-double tipping & non tipping. – Conventional tipping & non tipping. • Bulk Tippers: – B-double ...
www.kalari.com.au/loads/KalariBrochure.pdf

2003 () Trailer FELDBINDER FFB EUT49.3 POWDER TANK Triaxle Trailer Tankers FELDBINDER 2003 FFB EUT49.3 ALU. NON TIP POWDER TANK. 3 X HOPPER. ...
www.preloved.co.uk/fuseaction.../sec.../b4b9b5a9.html

Notes:- KIP 48.3 Tipping powder tank,Single front end tipping ram. BPW Tri-Air Alcoa, ... Body Type: non tip. Product: POWDER. Capacity: 49cm. Compartments: ...
www.intank.co.uk/sales.html


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Note added at 20 hrs (2009-11-06 07:29:57 GMT)
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Why not flatbed:

Available in 2 CONFIGURATIONS (DROPSIDE OR TIPPER), the Dyna FLATBED is the ultimate in practicality and safety. The FLATBED DROPSIDE has a fixed deck with drop down rear and side gates for easy loading and unloading, while the FLATBED TIPPER features a tipping deck that can tilt up to 50 degrees.
http://www.toyota-europe.com/cars/conversion/dyna_flatbed.as...