Each object has it's own controltip text, so it'll change depending on what object you're hovering over.
Anyway, you can always control the text by code by using the format:
Forms![object_name].ControlTipText = "Tooltip text goes here"
Now, if you're talking about hovering over say the left side of a button versus the right side of the button, there's no way of doing that. An object only recognizes IF it's been hovered over, not where.