What does the 'M' indicator stand for?

[tomczak]

There are 'S' and 'M' indicators in the Thumbnails caption. 'S' stands for the image being saved as a file, I gather. What does 'M' stand for? Modified?, but only after saving it first (i.e. there can not be M unless there was an S first)?

[jsachs]

M means that the image has been recalculated since it was saved and is therefore possibly, but not necessarily, modified since it was last saved.