Anyone who has ever had to look for a job knows what a job description is. What recruiters and HR folks know as a "JD", a job description is meant to be an overview of what the job opening requires, the type of work to be done, and the skills needed for the position. But in many companies, the JD is used as more of a "placeholder" to ensure that the position is being offered fairly to all applicants. Much like a rental agreement is no guarantee of a place to live, the company is in no way beholden to the JD as a definite outline of the job opening.
An HR friend of mine once told me that there are two internal uses for a job description; to help filter through all applicants for the most viable experience, and to help narrow down those viable applicants to one's who are aligned with the needs of the position. A job position is, after all, a need within the company, work that needs to get done which in most cases requires special skills to either accomplish the tasks or to get them done faster. But when we see a job title and description, what exactly do they mean to an applicant?
The job title can be misleading. A project manager, for example can be perceived as one who manages a project, oversees its progress, and maintains status and cross-functional communication. But for the company, the project manager might actually be supporting an entire division, or might be nothing more than a person who is supporting a small group of assemblers. HR folks will tend to skew the job title to align candidates with desired skills associated with the position.
The job description becomes the funnel for the job title, further describing the details behind what the company is really looking for in filling the opening. In most cases, the job description can be the tell-tale overview of what the company really wants. It really depends on how forthcoming the company is with their JD. Some companies will be very specific and detailed about their needs, while other companies use cookie-cutter JDs for use as a general placeholder for the position. In general, if a JD looks like a cut-and-paste document, then it probably won't tell you everything you really want to know about the job. The bottom line: Don't always believe what you read.
What this all boils down to, is a basic evaluation -- job needs compared to the budget for that position. A company will evaluate the needs of the position first, and then decide how much they can spend for that position. In some cases, the budget may limit the level of candidate who might be eligible for the job. For example, a senior technician may be used to fill the position for an associate engineer, based on the budget of the position. When decision time comes around, it will come down to what you have to offer versus your salary expectations.
Job descriptions can be confusing and even misleading. Companies looking for specific people to hire will use the JD as a filter, to help in picking out the most ideal candidates from the pile. The best way for you to use a job description, is to read between the lines and try to decipher the true job that lies underneath all of the official verbiage.
Somewhere in the job description is an actual job... your task is to figure out what the company really needs.