Anything you like from DC up to 2MHz, max voltage 2.5V according to the book, it's the external X input for use in X/Y mode, just with some limitations.
With DC you should be able to get full deflection of the dot either way, with positive one way, negative the other, center the dot with the X position first. Hopefully this will confirm if the X output amp is working.
P.S. Oh I was wrong yesterday about the switch position, the labelling is in for normal sweep & out for external X input.
David