BIK has it correct.
As for tour of duty vs duty period/time, a 'tour of duty' is a description of a particular type of activity - as defined in the CAO - while a 'duty period' is the time interval taken to engage in a tour of duty.
I'd suggest that 'tour of duty' gets a definition in the CAO because a specific set of activities are to be measured. The meaning of a 'period' in the normal English sense as per any dictionary is the meaning desired in CAO 48 - so no additional definition is given.