I have found the following somewhere in the documentation.

\button{...}[the chapter on whatever]
\button{...}[otherdoc::some topic]
\button{...}[previouspage]
\button{...}[PreviousJump]

I have tried them : all run very well except \button{...}[otherdoc::some topic]

How can I replace some topic to indicate a chapter in otherdoc or a page ?

Thank you.