This symbol  found next to an employee name while rostering (Job Maintenance), indicates a negative score and a warning to investigate further.  This is done by right clicking on the employee and selecting Score Details.

This symbol also found next to an employee name while rostering, indicates a failure to pass the rostering rules and therefore cannot be rostered, although this can be overridden by using the Master List button. Care needs to be taken to make sure there are no double ups created (same person on two jobs at the same time).