Well at present they're all handled individually - and when they need to be compared/aligned/managed they, as best I can tell, are literally done side by side
by eye with no field or form informing another.
Of course, it would be easy to say "we'd like it all to integrate!" but realistically, given the vast-knowledge-chasm we're working with, and an abject reluctance to spend money on training or invest any time in improvement, smaller, initial wins, are probably the first on order. But (obviously) it would be good if the functionality can scale in future.
So as a start;
Invoicing/Purchases is/will probably always be handled exclusively from timesheets etc - since our contracts aren't "strictly" time-based.
It would be nice to have annual leave (inc. requests for) and absences relate to the office calendar (even automate out-of-office?)
Resourcing would need to know about annual leave, but (probably) not tracked within the calendar
...with an eye for Project planning (what gets done when by whom) to talk to the calendar (office/project/user)