Reading isn't a school subject after age 8 at school - it is assumed that all children can read by then, and the focus moves on to studying literature - so I don't really know what people mean by "grade 6 reading" etc.
I can tell you what I meant. It was the general level (or levels, as mostly listed ranges) of the books my son read for pleasure combined with what I assigned. It seems important to list a general reading level in order to have an understanding of that aspect of the child's academic functioning.
Why?
(To be slightly more helpful: once decoding is no longer an issue, which I suppose is what DS's school is placing at 8 for a mixed-ability class, I would expect that the fiction a child enjoys reading is determined by sense of humour, presence of characters with whom the child enjoys empathising, degree of suspense enjoyed, etc.; the science factual material is determined by scientific background knowledge; the political opinion by exposure to political thought, etc. Lumping this together into a "reading level" seems unlikely to be practical or helpful.)
