The documentation on each display item in Visual Studio's Environment options for Fonts and Colors is lacking. There is a documentation page on msdn, but even this is incomplete (not all display items are listed), and contains limited descriptions. This makes it difficult for those that like to customize their code editor styling to understand which items they need to manipulate. The lack of a search function also further complicates things.
The main question which would answer this question is, is there more extended documentation on this available somewhere?
Since I highly doubt this (at least my search efforts have failed me thus far), I suggest to start bundling more elaborate documentation where needed (complicated display items) posted as answers to this question. It is encouraged to add screenshots so the visible impact is instantly visible.
There is no documentation available for Highlighted Definition, Highlighted Reference, and Highlighted Written Reference. These all pertain to highlighting similar items within source code to the item currently selected.
Highlighted Definition: Specifies the highlight color for where the currently selected item is defined (e.g., member definition, type definition, constructor, ...).
Highlighted Reference: Specifies the highlight color for where the currently selected item is referenced.
Highlighted Written Reference: Specifies the highlight color for where the currently selected item is assigned a new value.
In the example below: