Hey Vin,
I had a dig around with this a while back- it's worth setting up a SQL trace to show you which tables have been updated in your own environment, but generally a correction on the assignment form updates per_all_assignments_f and per_periods_of_service. I've still no idea why per_periods_of_service is updated, as it seems to simply enter a new row, with exactly the same details as the previous one- perhaps someone else on here may be able to shed some further light..
Cheers
Simon
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Simon Barrow