That is the purpose of the connector pane in your VIs. You can use the connector pane to define inputs and outputs for your subVI. Then, when you call the subVI from another VI, you can wire the data to the appropriate terminals. There must be a simple LabVIEW basics tutorial online that you can follow along with. You need to know this stuff if you plan on writing code that doesn't drive you insane.
Do a search on Functional Globals. It is a type of Action Engine that stores a value in the shift registers until it is changed within a state machine. The last value can be read by any other running vi. Values can also be written by any other vi.
