What does it mean to "Use rendered output to group rows?" Is the alternative to use raw output?
To what grouping is it referring?
Does this affect the sort order of rows?
Say you have a node with a taxonomy term, then a normal list would look like this


Title: A, Term 1
Title: B, Term 2
Title: C, Term 1
Title: D, Term 3
Title: E, Term 2
Title: F, Term 1
Title: G, Term 3


If you group them on the term, you would instead get something like this


Term 1


Title A
Title C
Title F

Term 2


Title B
Title E

Term 3


Title D
Title G



Rendered is not related to raw/safe values. It's just a way of explaining that it's the rendered (displayed) values that are used as grouping field.