Top 75 is listed as a 'hit' single. Top 100 is or was listed in NME up to recently. I have heard of singles having a position below 100 but don't know if this is official.
It's a top 200 from the offical chart people, my mate who works in a record company shows it to me once. It is very precise. At the bottom of the chart records are shifting as little as 50 copies.