This is a good question. The word "that" has so many usages in the English language. So, I can see you confusion. However, from a grammatical point of view the word "that" in this sentence functions as a demonstrative pronoun. this may not seem apparent at first glance, but under closer examination it becomes clearer. So, let me explain.
If I gave you the sentence, "He has done a job," you would probably ask, "which job?". At this point I would say either "this job," or "that job." The "this" or "that" specifies which job is in view. From a grammatical point of view this is called a demonstrative pronoun. In other words, it is demonstrating what is in view. It makes the job, in this context, not a general job, but a specific and concrete job.