Modification trails in editor

Hi.

Can anyone explain to me the modification marks on the right and left of the
editor?

Mainly I don't understand when they get reset. I have tried checking in the
file (it becomes read only) and closing and reopening, but the marks were
still there, but sometimes it goes away.

As a sidenote, I just love the fact I have syntax coloring and completion in
expression evaluation during debugging!

Thanks,
Amnon