The middle tier consists of engineers with high talent but no experience, or less talented but lots of experience. It still requires more talent than the general population (computer science is peculiar and abstract and it’s not easy for the general population). I count myself in this tier. These are the implementers that build out the innovations designed by the top tier. This tier is vital to a company’s long-term success, because ideas need to be put into practice and operation to be useful to enough people to make money.
The bottom tier is little talent and little experience, or brand new graduates that might move up the tiers with a little experience. They’re doing the scut work that isn’t worth using the higher tiers to do.