Trying to understand different icons in Hierarchy

I see three different icons in the Hierarchy for GameObject's and Prefabs. Is there a document that explains what the different icons represent?

1. Blue Cube = Prefab Instance
2. Gray Cube = GameObject Instance
3. Gray Cube with plus symbol = ?

Side note:
When I saw the the gray cube icon for the first time, I thought it indicates these objects are turned off.

 

Plus symbol -> this object is added to a prefab instance

 

+1

 

I'd like to see the editor team pay more attention to the details, follow long established convention and respect the way the human brain perceives information.

The Hierarchy looks busy with extra visual complexity, this is caused by the display of gray boxes, this is unnecessary because the information is redundant with no icon
The icons are on the immediate left of the text so all relevant information (which is text) is offset, this forces the eye to skip complex visuals (icon) to access data
Conventions are violated: gray icons which indicated disabled objects are used
The information density doesn't match the added complexity of nested prefab because all the prefab box icons are blue, occluding the nesting depth information

Solutions
Remove gray icons and only show icons when the information is useful (+box, blue box)
Move the icon to the left of the arrow
Color the boxes with increased saturation of blue or different colors indicating nesting depth
Color the text blue