<p>Companies that are US military contractors often work on projects that entail some data that is classified. If you are assigned to such a project, and will need access to any of that data, your company will put you through the process of getting a security clearance. You fill out a lot of detailed forms about yourself and a govt agency does a background check; they’ll interview people you know, maybe yourself. It may take months. If you are denied, then you can’t work with classified data (but could be put on another project that doesn’t require the clearance). They do a re-investigation every 5 or 10 years if your clearance needs to stay active. </p>
<p>As for ‘consultants’, they are often contract engineers, who are contracted out to other companies by the consulting company. A project may have, say, 90% direct engineers and 10% contract engineers who all work together to accomplish a project. (The contractors don’t necessarily have special skills that the company otherwise lacks.) The benefit for the compay is that contractors can buffer their headcount - it’s easier to let them go when headcount needs are lower. The benefit to the contractor is they are paid more. My company has some contract engrs. that have been with us for many, many years. Most of the ones I know are software engineers.</p>