In the FrameMaker help the different file icons in a book file are not explained

FrameMaker 12 and 2015

A colleague inherited a book file with some file icons in red, and

he did not know what this means.

I found out that the file icons turn red, when "Exclude" is activated.

I had also tried to find an explanation in the FrameMaker help,

but there isn't anything about the file icons in book files.

I know that in this forum there are also questions about TOC or

index files which do not update, and the cause is often

that these are only regular files and not generated files.

When these icons are explained, then at least those who

check the help could find the answer themselves.