I think that "related field" depends on the job description. Personally, I would look at not only the degrees held by the candidate, but also their minors, tracks, and work experience to try to determine if they have some combination of classroom experience and practical work experience in the domain that I'm hiring for.
For example, for an embedded systems engineer, that could mean the candidate could hold a degree in software engineering or computer science (especially with concentrations in computer or electrical engineering), computer engineering, or electrical engineering (with some low-level programming experience). I probably wouldn't hire an IT or Management Information Systems graduate for this position. However, for building an enterprise application system, I probably wouldn't consider a computer or electrical engineer who has most of their classroom and/or work experience in lower-level programming. And for another position, an algorithm developer, I would look toward computer science, software engineering, or mathematics with a programming background.
From an HR perspective, it's a catch phrase used to allow people who have graduated from different degree programs to have opportunities at jobs they are qualified for. Different universities call different programs different things. By saying "or related field" in a job posting, it opens the door to letting the company look at every qualified applicant, without regard for what their degree says on it.