In the context of career choice, a person's replaceability is the degree to which their role impact would vary should that position be occupied by the next most likely candidate.
Benjamin Todd illustrates this idea with an example:[1]
Suppose you become a surgeon and perform 100 life saving operations. Naively it seems like your impact is to save 100 people’s lives. If you hadn’t taken the job, however, someone else likely would have taken it instead. So your true (counterfactual) impact is less than the good you do directly.
This type of consideration is relevant not only to careers, but to donations (“Would someone else have fulfilled charity X’s funding gap if I hadn’t?”) and fundraising (“If I persuade someone to give money, would they have given it anyway?”).
However, it is often unclear to what an extent replaceability applies for a given action, and 80,000 Hours notes that for many career decisions replaceability has only limited relevance.