Let's say that you are constructing a technical employment advertisement and you would like to specify that this position requires someone who knows what they are doing in a few technologies.
A typical want ad will say something about required years of experience in each of those technologies. This is bad because it is not merely imprecise, but it is a proxy for what you actually want. Clear communication requires that you ask for what you want, not something that approximates it.
(Also, asking for specific years of experience might open you to age discrimination lawsuits in parts of the world where that's a thing. I am not a lawyer.)
Let me offer you this scale, instead:
Beginning: has started to use this technology
Familiar: has successfully completed at least one major project using this technology
Proficient: routinely uses this technology in many projects
Advanced: has taught others; is comfortable discussing design and development of the technology itself
Expert: generally recognized and associated with the technology
Each of these stages represents a clear jump in competence. Which would you rather have: a person who has read this scale and declares that they are familiar with your primary programming language, or someone who read "3-5 years of Ruby" and decided that because they have encountered Ruby once or twice a year for each of the last four years, that fits? In the other direction, people can acquire significant skills in short periods of time if the conditions are right. There are technologies where it is possible, given the right conditions, to gain proficiency in a year or two.
One might also be interested in recency: I have beginner-level competency in a bunch of languages that I haven't used in ten years. I'd like to think that it would only be a matter of a week or two to regain those skills, but they have certainly rusted. I might drop them from my resume, or leave them on since the jobs I like tend to be interested in generalists who learn things quickly.
It is rare for a position to demand an Expert in anything; it's more likely that an Expert is part of your organization's founders or is approached personally, rather than through an open advertisement.
This post was generally inspired by Hiring Dark Pattern