Comma in relative clause
The question about commas in English has always been really difficult for me. I was told in school, that commas are not that important in English meaning that there are not so strict rules for using commas (compared to German). This confuses me all the time. For example:
I have the following sentence:
The modules , used by the software tool , are introduced in this chapter.
Would you use both of the commas?
If the relative clause does not provide information necessary to the meaning of the sentence, do separate with commas:
"Jim, who shares my love of writing, helped me edit this book."
Here, "who shares my love of writing" is information that's nice to know--perhaps even helpful as context--but it's not crucial to the meaning of the rest of the sentence. "Jim helped me edit this book" can be understood all the same without this relative clause. Thus, we separate this relative clause with commas.
If the relative clause provides necessary information for the sentence to make sense, then don't separate the clause with commas:
"Every criminal who has been convicted of murder must be put behind bars."
Here, "who has been convicted of murder" is essential to the meaning of the sentence. It would be a gross misreading of the sentence to gloss over this relative clause by reading: "Every criminal must be put behind bars."
Let's look at this now:
The modules, [which are] used by the software tool, are introduced in this chapter.
Is "used by the software tool" a friendly tip to remind your reader that the modules happen to be used by the software tool? If so, you probably want to use commas. But if this clause is a way of specifying specific modules, and without such specification the sentence means something different, then don't use commas. Your case, I think, is a matter of context and intended meaning.
So, for example, the sentence could be employed in two different ways, one which requires commas and another that doesn't.
With commas (relative clause is not necessary for basic understanding of the sentence):
Welcome to my guide on CSS modules. The modules, used by the software tool, are introduced in this chapter.
Without commas (relative clause is necessary for basic understanding of the sentence):
In this book, I will introduce two types of modules in our products: the modules used by the hardware and the modules used by the software tool. The modules used by the software tool are introduced in this chapter.
Some other examples:
"Blessed is the man who listens to me." Proverbs 8:34
"Blessed is the one who does not walk in step with the wicked." Psalm 1:1
"You must not eat fruit from the tree that is in the middle of the garden.” Genesis 3:3
It gets trickier, though. Sometimes you may not want to use commas for very short relative clauses, just because cutting a sentence into many short segments may not be aesthetically pleasing. What's more, in many cases the necessity of the information in the relative clause may be quite ambiguous. The tone of speech may also factor into your decision to use or not use commas: do you want to put short pauses in your sentence where the commas go? Moreover, when you don't write out the relative pronoun (which, who, that) and only imply it, the requirement to use commas may loosen somewhat. Consider the following more ambiguous cases:
"Tell me, you whom I love, where you graze your flock and where you rest your sheep at midday." Song of Songs 1:7
"Then God said, 'Take your son, your only son, whom you love--Isaac--and go to the region of Moriah.'" Genesis 22:2
Genesis 3:11 is translated with and without a comma separating the relative clause, depending on the translation:
NIV "Have you eaten from the tree that I commanded you not to eat from?"
KJV, JPS (1917) "Hast thou eaten of the tree, whereof I commanded thee that thou shouldest not eat?"
So, basically, treat everything I said with a grain of salt. It's useful as a rule of thumb, but it's not an especially strict convention. Hope this helps!