check "partial ordering" concept. What is the minimum independent "thing"? Probably user?
example over user+invoices:
i.e. there are things that have to come in exact order (e.g. activity on certain invoice), and there are things that can move around (i.e. processing those, timewise), being independent from one another (different invoices' activities, wholesale). But when same user acts on different invoices, then whole one-user-activity should be in exact order.. not just invoice-activity
example over user+invoices: i.e. there are things that have to come in exact order (e.g. activity on certain invoice), and there are things that can move around (i.e. processing those, timewise), being independent from one another (different invoices' activities, wholesale). But when same user acts on different invoices, then whole one-user-activity should be in exact order.. not just invoice-activity