Examples:
* Without comma: The company's profits were a result of the success of its new product line, the sales thereof exceeding expectations.
* With comma: The company's profits were a result of the success of its new product line, the sales thereof, which exceeded expectations.
Explanation:
In the first example, "thereof" simply means "of it" and is not separated from the rest of the sentence by a comma. In the second example, "which exceeded expectations" is a non-restrictive clause that adds additional information but is not essential to the meaning of the sentence. This clause is separated from the rest of the sentence by commas.
In general, "thereof" is often considered outdated or overly formal. It's usually better to rephrase the sentence using more common language.
For instance, you could rewrite the above examples as follows:
* The company's profits were a result of the success of its new product line, with sales exceeding expectations.
* The company's profits were a result of the success of its new product line, and these sales exceeded expectations.