下面这段话中的“commonly known as PCs.”是过去分词作后置定语，修饰“computers”吗？

Question

Though these large machines still exist, many of their functions have been taken over by small powerful personal computers, commonly known as PCs.（虽然这种大计算机仍然存在，但是它们的许多功能已被体积小但功能齐全的个人电脑——即我们常说的PC机——所代替了。）——《新概念英语》
那为什么要加逗号呢？这个逗号一定要加吗？

Answer 1

你说的全对，加逗号是因为这个定语相当于非限制性可有可无。which are commonly known as PCs.追问

您好，但是不是只有从句才分为限制性定语从句和非限制性定语从句吗