Well, our school district has GT (gifted and talented) teachers at every school at least one day per week. Used to be more, went down with budget cuts. Anyway, the GT teachers do enrichment and pullouts with generally the top 10% of kids at every school in our district, as identified by the required testing in 2nd grade.
Our district then has an HGT (highly gifted and talented) program that serves only the children testing top 2%. Those children can go to one of five elementary schools and a single middle school that have a magnet HGT program colocated with a regular school. You cannot attend one of those programs unless you test in the top 2%.
Regarding how narrowly we define giftedness for the sake of explaining to others that gifted kids within whatever range have more severe needs than kids at a slightly lower range, I have a lot of thoughts that I can't put together coherently right now. I agree with ultramarina. I think that HG and PG kids are vastly different from MG kids, based on reading posts on this forum over the past year. In that sense, maybe even the top 2% definition is too generous. My top 2%-er is doing fine in a regular public school classroom with subject acceleration in math and grade level ability grouping in reading; but, we're at an academically rigorous school with bright kids and an administration that embraces subject acceleration and ability grouping; and we're about to screen for dyslexia.
I think everyone falls on a continuum and it's very difficult to draw a line, but I read the posts from those of you with HG and PG children, and you have to deal with a whole host of issues that I don't have to deal with, thus far, with my MG children. In that sense, and to respond to the OP's requests, perhaps the more narrow the definition, the easier it is for parents of very highly gifted children to explain to educators what they're dealing with; but only if educators understand the differences in the first place.