关区：一般是指海关总署根据全国各地的具体需要和综合考量，划分的不同区域，在这些区域上，可以进行货物进出口等一切与之有关系的活动 （https://zhidao.baidu.com/question/396038752066832845.html#:~...）

口径：网上只有“经济口径” 的定义，但这个定义在你应用的上下文当中说不通。找到了一篇有关2019年天津外贸统计的文章。这里对口径的说明应该符合你的情况：”外贸进出口总额 指海关统计中按经营单位即进出口企业在海关注册地的行政区域口径统计的数据，它反映的是天津行政辖区内各类具有进出口经营权企业（外贸企业）的进出口。它不包含外省市外贸企业途经天津口岸由天津海关结关放行及统计的进出口商品，但包含天津外贸企业经由非天津口岸进出口结关放行及统计的商品” (chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/viewer.html?pdfurl=http%3A%2F%2Fstats.tj.gov.cn%2Fnianjian%2F2019nj%2Fzk%2Fhtml%2Fzb11.pdf&clen=194202&chunk=true).

按这个说明推理，你问题的中的关区口径，应该指某个贸易区所有外贸企业的进出口额。

关区的英文翻译没有太大把握，不知是否能翻译为 "Customs Territory".

关区口径试译："all the import & export companies in the customs territory," 或者 "all the international trading companies in the customs terrioty." 仅供参考。

按关区口径，进出口额达XXX万亿元、增长XX%。
According to the customs area ports (authorities/statistics), the import and export volume reached XXX trillion yuan, an increase of XX%.

A customs area is an area designated for storage of commercial goods that have not yet cleared customs. It is surrounded by a customs border. Most international airports and harbours have designated customs areas, sometimes covering the whole facility and including extensive storage warehouses. For the purpose of customs duties, goods within the customs area are treated as being outside the country.
https://www.definitions.net/definition/CUSTOMS AREA

“口径” 可能是进出口货物吞吐的途径（港口，机场，边境关闸）。
The terms “port” and “port of entry” refer to any place designated by Executive Order of the President, by order of the Secretary of the Treasury, or by Act of Congress, at which a Customs officer is authorized to accept entries of merchandise to collect duties, and to enforce the various provisions of the Customs and navigation laws.
https://www.govinfo.gov/content/pkg/CFR-2008-title19-vol1/xm...