I think it refers to reforms of preferential access, resulting in reduction of the value of these preferences. Please read the following context for reference:
· Mauritius because of an agreement to prevent double taxation. Tapping other markets: Mauritius, located off the southeast coast of Africa, is expected to lose foreign revenue with the end of preferential access to lucrative western markets that its sugar and textiles exports have enjoyed for over 30 years. It now wants to make more inroads in India, and provide it with a base to explore markets in ...
www.bilaterals.org/article.php3?id_article
· ... of Preferences (known as GSP for short) is a scheme whereby a wide range of industrial and agricultural products originating in certain developing countries are given preferential access to the markets of the European Union. Preferential treatment is given in the form of reduced or zero rates of customs duties. The GSP scheme is specifically designed to ...
www.revenue.ie/services/customs/origin_info.htm
· Namibian Government on preferential market access The Permanent Secretary in the Ministry of Trade and Industry, Andrew Ndishishi says Namibia is still eligible for preferential market access under AGOA, SADC Trade Protocol and the Cotonou Agreement. Citing the government position on the recent closure of the Rhino Garments Factory that left 1 600 people jobless, Ndishishi noted ...
www.grnnet.gov.na/News/Archive/2005/may/week2/market.htm

ㄑ把σ

外贸常用英语词汇(进出口贸易)，给需要的朋友 - [ Translate this page ]
国际贸易值value of international trade 6park.com 普遍优惠制generalized system
of preferences-GSP 6park.com 最惠国待遇most-favored nation treatment-MFNT
6park.com 6park.com -------------------价格条件---------------------- 6park.com ...
www.6park.com/life4/messages/90738.html - 22k - Cached - Similar pages

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Note added at 7 hrs 4 mins (2005-11-06 01:42:03 GMT)
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TITLE - [ Translate this page ]发展中国家全球贸易优惠制（Global System of Trade Preferences Among Developing Countries，简称：GSTP）是由77国集团于1982年倡议，在发展中国家间开展的南南诸边 贸易全球性经济合作项目，其目的是通过关税减让、消除关税和非关税壁垒、实施中长期 ...
genevese.mofcom.gov.cn/aarticle/ jmjg/zwqtjmjg/200412/20041200313250.html - 16k - Cached - Similar pages


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Note added at 7 hrs 7 mins (2005-11-06 01:45:09 GMT)
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关税种类 - [ Translate this page ]
最不发达国家（简称LDC）和东帝汶（ETIM）, 零关税； - 发展中国家和地区（简称：DC、DCS
和DCT），关税低于一般税率，但高于FI关税 ... 根据双边贸易协议的规定，对新西兰、
巴布亚新几内亚原产的大多数商品免税， 对加拿大原产的某些货物也给予优惠税率； ...
www.auwintl.com/Chinese/business/ ACS&AQIS/types_of_Duties.htm - 10k - Cached - Similar pages



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Note added at 7 hrs 9 mins (2005-11-06 01:47:30 GMT)
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以我了解preference 一般指 preferential rate of duty = 优惠税率

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Note added at 7 hrs 13 mins (2005-11-06 01:51:31 GMT)
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外贸关税术语释义::国际营销传播网 - [ Translate this page ]
最惠国税率(The Most-favoured-nation Rate of Duty) 优惠差额（Margin of Preference)
优惠税率（Preferential Rate) 有效保护率（Effective Vate of Protection) ...
www.globalmarketing.cn/ Article/operation/200504/1901.html - 65k - Cached - Similar pag

[you can see subsidies,price supports and tariffs 进口税率 are about (国内生产)保护... ]

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Note added at 14 hrs 33 mins (2005-11-06 09:11:40 GMT)
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value = 用处;价值(朗文进阶词典)

FYI