They have introduced a new system whereby all employees must undergo regular training. 

参考译文：他们采用了新的制度，所有的雇员都必须定期进行培训。

whereby (关系代词)凭此;借以 (英语解释A system or action whereby something happens is one that makes that thing happen)

the system whereby Britons choose their family doctors and the government pays those doctors.

通过这个系统，英国人选择自己的家庭医生，而由政府给那些医生支付薪水

They voted to accept a deal whereby the union will receive nearly three-quarters of a million pounds from the International Miners Organisation. 

他们表决同意了一项协议，根据这一协议工会将从国际矿工组织得到近75万英镑。

They have introduced a new system whereby all employees must undergo regular training. 

他们采用了新的制度，所有的雇员都必须定期进行培训。

1. They have introduced a new system whereby all employees must undergo regular training.

【翻译】他们引进了一项新制度，（凭借此制度）所有员工都必须定期接受培训。

这是一个完整的句子。whereby 是一个比较特殊的关系代词，意思和用法相当于 by which，即：凭借先行词所指的内容，是比较正式的用语。

这说明，all employees must undergo regular training 是a new system 的定语从句。

2. a proposal whereby EU citizens would be allowed to reside anywhere in the EU

【翻译】一项允许欧盟公民在欧盟任何地方居住的提案

这是一个名词词组。 EU citizens would be allowed to reside anywhere in the EU 也是a proposal 的定语从句。凭借此提案，欧盟公民在欧盟任何地方居住，就可得以允许。