There are a large number of companies that require ids as the basis for every selector, by policy. This may not be interesting to you, but I think this is what they meant. The fact they may or may not be familiar to you is incidental.
I'm familiar with policies that generally strive for specificity of (0, 1, 0) like BEM or (1, 0, 0), but I'd be very interested in any policy that rules out pseudo-classes like `:hover`, as prohibiting them would leave you to handle any interactivity you'd get for free from CSS via JavaScript.
