Assigning Time types to absences

Hi All,

I would like to know that is it mandatory to assign time types to absences? If no ,then In which scenarios Time types are assigned to absences. Also it would be great if someone could explain about table T555Y (In case of negative time management). Using F1 help I am not able to understand the documentation on T555Y .

Let say ,I am taking one scenario in which we are using negative time and only absences are recorded then what is the process flow  through which time type will get assigned to the planned working time (say 8 hrs.).

Thanks,

Gopal