Are sitemap and high-level sitemap the same thing?
I tried to search for it, but I can't seem to find a difference. Only a few of sites actually call it high-level sitemap.
A sitemap hierarchy can involve many levels. It won't be unimaginable to find a sitemap made of 6 levels (although not all are navigation levels).
A high-level sitemap usually involves the first level, and possibly the second one as well.
Being top level, these are somewhat the more critical levels - as it is imperative they are mapped to user needs and land themselves well to the grouping of lower levels.
The following would be considered hi-level sitemap, with lower levels collapsed (the + buttons will expand them).