In a nutshell again: 1.man-hours & 2.elapse hours: 3 mechs work 8 hours - 24 man hours in 8 elapsed hours. 3.task hours - similar to elapsed hours. 4.labour - in terms of above - when a mechanic clocks on and off a job.
Part purchasing is an overhead function - it is not billed by the hour.
Also note - the hours suggested in SB using the above terms are never accurate - you usually have to double them.