Let's say you have this rule:
  when
     $e : Employee() // stupid example, you'd just nest that in the next line which is faster
     ShiftAssignment(employee == $e, ...)
  then
    ...

In that case, the justificationList would contain an Employee and a ShiftAssignment, for example [employeeAnn, shiftAssignment745].
So if you cast shiftAssignment745 to (ShiftAssignment), you could do getPost().getPostGroup().getCustomer() etc.

However, if you got
  when
     $e : Employee()
     accumulate(ShiftAssignment(employee == $e, ...); $s : sum(...); $s > 10)
  then
    ...

then the justificationList only contains an Employee, so for example [employeeAnn].