Value to the Left of "Frames" in Movie File Textures

When using a movie file as a material’s texture, under the “Image” panel it has the
“Frames” field (ImageUser.frame_duration, “Number of images of a movie to use”). To the left of the value in the field is another number in parentheses.

What does this number represent, and by what logic is it calculated? The frame that is being shown in the editor, perhaps? It seems to be affected by the values below it, but elusively:

When “Start” is at “0”, it is incremented to 1. When “Start” is anything positive, it is decremented to -1. When “Start” is negative, it is incremented to Start’s absolute value, and sometimes, perhaps influenced by a non-zero value of “Offset”, twice that. Then to this, the value of “Offset” seems to be added.

Are there docs on it somewhere, or is it time to go digging into the source?