Different Designs in the red close button?

[GuitarG20]

If i mouse over it in chrome, it's a x logo. But if I do that in word, it's a circle icon. what do these different icons mean inside the red close button?

 

[PurrBall]

A circle indicates that a document has been modified since it was last saved (or never saved).

 

[CK1Czar]

Cool info. Never noticed that