If you take one image away from this course, make it this one. It is worth drawing from memory, on paper, badly, with the video paused — the drawing is what fixes it.
Person. The human being. One record per human, ever. Sit with "ever": if somebody joins, leaves, and comes back four years later, that is one person record, not two. The person record holds what is true of them as a human — name, date of birth, national identifier, personal contact details. It does not hold their job, and keeping those two things apart is the entire point of the design.
Work relationship. The person's relationship to a legal employer. Its types behave differently and the names matter: employee, the standard employment relationship; contingent worker, engaged rather than employed; non-worker, connected without working in the usual sense — a pensioner, a board member, a volunteer; and pending worker, somebody who will start and has not yet, so you can prepare a record and run pre-boarding without them counting as live workforce.
Assignment. The actual job being done — department, job, position, grade, location, manager, working hours, full-time equivalent. When you are unsure where a piece of employment information belongs, ask whether it describes the human, the relationship with an employer, or the specific job. Three layers, three answers.
Why three rather than one flat record? Because reality is not shaped like that. One person, employed by your UK entity full time and separately engaged by your Irish entity as a contractor two days a month, is one person, two work relationships, at least two assignments. Model that flat and you are choosing which of two truths to store — and whichever you choose, something downstream is wrong: payroll pays the wrong entity, or the Irish headcount return is missing a contractor, or the person appears to have resigned in the UK when the Irish engagement ended. And one work relationship can itself carry more than one assignment, which is why "how many assignments does this person have?" has a non-obvious answer.
Now the payoff. Department, job, position, grade, location — every single one of them attaches to the assignment. Not to the person, not to the work relationship. That is why somebody with two assignments can be in two departments at once, why transferring somebody is an assignment-level action, and why headcount reporting has to be explicit about whether it counts people, relationships or assignments — three different numbers.
Three more objects sit across the top. Legislative data group partitions data by legislation and is payroll's foundation. Legal employer owns the work relationship and is what statutory reporting runs off. Business unit scopes reference data and security, and is operational rather than legal. They are separate on purpose: a legal employer is not a business unit, and a business unit is not a department. Conflate them and you get a design that cannot express something the business needs — discovered about two weeks after it becomes expensive.