Four components of a rhetorical situation: The Text, The Context, The Reader, and The Writer.
The text refers to simply the content as a whole, the words used, any images used, what form you write it in, and the media with which you use to communicate it.
The context is a much broader term, it can refer to the mood the reader is when he's reading it, the mood the writer is in when writing. It takes into account the relationship you have with the reader, how much he likes you, how much he knows about you, anything that effects the readers opinion of you, prior to reader your essay, and anything that effects the readers receptiveness (or non receptiveness.
The reader and writer both have similar explanations: It takes into account everything that they previously have learned, felt, desired, and experienced prior to reading your message.
The rhetorical situation of a job application is one that, most if not all of us have whether consciously or subconsciously thought through and applied. Before writing a job application and your resume you want to select the right form. For general purposes it is accepted that most applications and resumes should be very simple text, neutral in color, and very formal.
However, context can play a huge role in this, and there are many exceptions in which you would want to stray from the norm. Are you applying for a graphic design job? Add some flare to showcase some of your work right off the back. Marketing Job? Add some more personal touches, and flashier writing and form so that you stand out and promote yourself. Just like you would be promoting something for them.
While knowledge of the reader is not necessary or always available, if you know who is going to be reading your application then perhaps you can gear it towards them specifically. Perhaps add some things to your resume that would spark the interest of your reader due to a shared experience, or affiliation. However, this can be a double edged sword as you might add something that the reader has had a bad experience with by accident.