The profession offers a bundle of benefits, not least of which is extensive training.
参考译文：这份职业提供了大量福利，其中很重要的一项就是全面训练。
not least of which=a very important one of which ,which 为关系代词引导非限制性定语，which指代前面先行词benefits，并在定语从句作介词of的宾语，属于个体/部分与整体之间的关系。one=benefit(备注：least 意思“不重要，微不足道”，not least 意思“很重要的”）(not least在语义上相当于名词=an important benefit，很重要的一项福利）
The profession offers a bundle of benefits, not least of which is extensive training.
=The profession offers a bundle of benefits and a very important one of them is extensive training.
总结：提问网友理解是正确的。
类似以上介词of+关系代词which(of which)结构引导定语从句例句其它例如觉如下(of which 都是表示部分/个体与整体之间关系)
Many theories on punishment exist, all of which are variations on a theme.(which 指代 theories)
关于惩罚的理论有很多，虽形式略有不同，但其主旨都是一样的。
The house had its charms, not the least of which was the furniture that came with it..(which 指代 charms)
这座房子有其吸引人的地方，比如和房子配套的家具就很出彩。
There are three conference rooms each of which is equipped with a screen,flipchart and audio visual equipment.
有3个会议室，每个会议室都配有屏幕、活动挂图板和视听设备。.(which 指代 rooms)
I saw three camp beds, two of which were occupied.我看见了3张折叠床，其中两张已有人占了。.(which 指代 beds)
He has a shelf full of things, each of which has associations for him他的一个搁架上摆满了东西，每一样都能勾起他许多回忆。
