Outlook/exchange uses the former approach, a pattern with a list of instance-specific exceptions. One reservation system I worked on used the latter approach, w

Outlook/exchange uses the former approach, a pattern with a list of instance-specific exceptions. One reservation system I worked on used the latter approach, with every instance saved to the database. I was tasked with building an outlook plugin to smoothly integrate the two, and it was probably the most difficult piece of software to get working reliably I ever wrote. The prototype took a few days, the fully reliable version compatible with all outlook versions took two years. Outlook’s internals are pure madness.