In general, the contract is phrased as "Work that relates to activities and business that the employer is currently involved in, or may become involved in in the future". So if you work for a large company that has many businesses, almost anything you invent could be related to some area they are involved in. However enforcement is relatively rare.

Edit: I found an article on Slashdot that I remember reading at the time, which scared the crap out of me. About a programmer that made a bunch of GPL code on his own time, but his employer stole it back. https://developers.slashdot.org/story/02/03/21/0139244/bewar...