Think from the perspective of the VI, not the "program": a control is always an input to the VI on whose front panel it's placed, and an indicator is always an output. Whether the control or indicator is displayed to the user or not is really secondary.
If you're using a control to both pass data into your subVI and display it to the user, you may want to disable it so the user can't alter the displayed value - that might not affect the operation of your program but it could be confusing! The alternative would be to use a numeric control offscreen as the input terminal to your subVI and wire the value from that control directly into an onscreen tank indicator.
