Article 304(1) shall not apply in respect of existing Mexican waivers of customs duties, except that:
(a) Mexico shall not increase the ratio of customs duties waived to customs duties owed relative to the performance required under any such waiver; or
(b) Mexico shall not add any type of good to those qualifying on July 1, 1991, in respect of any waiver of customs duties in effect on that date.

Article 304(1) shall not apply in respect of existing Mexican waivers of customs duties, except that:
(a) Mexico shall not increase the ratio of customs duties waived to customs duties owed relative to the performance required under any such waiver; or
(b) Mexico shall not add any type of good to those qualifying on July 1, 1991, in respect of any waiver of customs duties in effect on that date.
条款304(1)不适用于现有墨西哥关税减免，除下述情况外，：
a)墨西哥不应增加海关税减免与海关应收之比例，除执行(本协议)所要求的任何减免外；或
b)除1991年7月1日起生效的任何减免关税项目外，墨西哥不应增加任何货物类别。

这是我个人理解，墨西哥那时候要求尽量减少延缓进口免税项目的意思。