JamPlus manual
Warnings and Errors

In addition to generic error messages, jam may emit one of the following:

warning: unknown rule X
   A rule was invoked that has not been defined with an "actions" or "rule" statement.
using N temp target(s)
   Targets marked as being temporary (but nonetheless present) have been found.
updating N target(s)
   Targets are out-of-date and will be updated.
can't find N target(s)
   Source files can't be found and there are no actions to create them.
can't make N target(s)
   Due to sources not being found, other targets cannot be made.
warning: X depends on itself
   A target depends on itself either directly or through its sources.
don't know how to make X
   A target is not present and no actions have been defined to create it.
X skipped for lack of Y
   A source failed to build, and thus a target cannot be built.
warning: using independent target X
   A target that is not a dependency of any other target is being referenced with \$(<) or \$(>).
X removed
   <b>Jam</b>  removed a  partially built target after being interrupted.