The objective, since attained, was to create a set of contractual rules that would establish uniformity in that practice, so that practitioners would not have to cope with a plethora of often conflicting national regulations.

        since attained作什么成份？

http://ask.yygrammar.com/q-34521.html (第一帖)

http://ask.yygrammar.com/q-34535.html （第二贴）

▲在第一帖中，freeone老师的观点是：

since attained = which was attained since（后来实现的……）

▲在第二贴中，曹老师的观点是：

既然since作为介词不可后接过去分词，我们就没有必要再考虑介词或连词。我赞同蒋老师（freeone) 的分析，此时的since是个副词。

The objective, since attained, was to create a set of contractual rules that...

= The objective, which had since been attained, was to create a set of contractual rules that...

since指前面提及的一个时间或事件，主语the objective是在那个时间或事件之后才制定的。本句中主句谓语为一般过去时was, 也可以证明since并不是修饰主句谓语的。

我对比一下两位老师的观点，发现存在差异：

freeone老师认为，since attained = which was attained since（后来实现的……）

曹老师认为, since attained= which had since been attained

▲我倾向于freeone老师的观点，并提供一下理据：

  朗文

since : at a time after a particular time in the past 此後；後來

例如：

We were divorced two years ago and she has since remarried. 我們兩年前離了婚，之後她又再婚了。

柯林斯

ADV  副词(表示过去某事或某情况发生后不久又发生了另一件事)此后，后来 When you are talking about an event or situation in the past, you use since to indicate that another event happened at some point later in time.

【搭配模式】：ADV with v

About six thousand people were arrested, several hundred of whom have since been released...

约 6,000 人被捕，后来其中的几百人又被释放了。

因此，我认为：since是副词 

The objective, since attained, was to create a set of contractual rules that...

= The objective, which would be / was to be attained later, was to create a set of contractual rules that...

其中since是副词，since attained是非谓语动词作非限制性定语，意思“后来得以实现”。

另，attain—succeed in achieving (something that one desires and has worked for) 获得；实现

clarify your objectives and ways of attaining them.说明你的目标和实现目标的方法

请老师们发表看法，把这个为弄透彻。