What does the stained designation on a coin indicate?

I saw this coin recently, proof trade $, https://www.ebay.com/itm/204427729130?
that has a stained designation, but to me looks like toning, I was curious how can differentiate stain versus toning? Also how does one even stain a coin? Is staining the same as artificial toning?

 

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I am not sure what the grading services mean by staining.
Personally, I saw a group of 4 Seated Dollars in a coin shop in Sioux Falls South Dakota back in the 1990's. All four had been stained with some sort of walnut wood stain. You could even smell it. I said nothing and never went back. James

 

I would assume that "stained" refers to a foreign substance adhered or adsorbed onto the coin. "Toned" refers to a layer of oxide or sulfide produced by a chemical reaction with the coin's surface. I'm not sure how sharp that dividing line really is, though.

 

The toning on that coin has gone too far. The surfaces under the black toning are probably dull with no mirrored reflectivity.

 

Ugly toning. It should be professionally cleaned and treated.

 

Here's an example of staining (not toning). Something contacted the surface of this OBW end-roll coin in a circular pattern. You can see, the coin is toned and that looks quite different from the staining. The stain is on top of the toning.