When Tiffy shows Col. Ryan a letter in the beginning of the episode, she refers to it as "Mac's Re-enlistment". The letter is titled "Notice of Re-Enlistment" with the name, GERHARDT, MACDONALD, etc....
The body of the letter however, contains 3 paragraphs of text that basically explain why he needs to get out of going to active duty. The letter says that he is in the delayed entry program, which precedes basic training. It also explains that his mother's arthritic condition has worsened considerably and that he is the oldest of 3 of three children, having two younger brothers, ages 7 and 12, that he has to help his mother with. It also explains that he has changed his career plans and wants to be a graphic artist.
When Mac hands Bob a cup of coffee, Bob comments that "coffee's for closers," deliberately quoting a famous line from the film/play "Glengarry Glen Ross". David Mamet, creator of "The Unit" and director of this episode, wrote "Glengarry Glen Ross".