This is the old-time accounting part of your accounting system; the part that computers and software cannot replace. This is also where you can see the difference between a bookkeeper, an accountant, and a management-level accountant.
The former knows the processes as learned, without understanding underlying purposes and theory. The second knows the accounting theory, but lacks the high-level experience to see the big picture: to handle the changes that growth requires.
So a system review is a check of whether policies and procedures are in place for the level you are at and plan to be soon. This includes checking whether your procedures have the right levels of audit-trails in place and enforced, whether they have proper separation of duties; whether things are done in the proper order. Are reviews and approval processes in place to ensure things are happening like you believe they are happening?