Format Checkbox so that the "check" can be a "check mark" or an "X"

There is no way to turn a checkbox (two logic states, true or false) into an X, a check, and a blank (three logic states).

There is no way to have the check display as an X instead without getting into the guts of the application. I think you can just change the png files that represent a checked and unchecked box. Find the Numbers app, right click, Open Package Content. In it are several png files for the boxes in the Contents/Resources folder. I don't know which ones are which but my first guess would be all the ones that begin with TSD_TableViewCheckboximage . I don't feel like experimenting but you can if you want to. Back up the original first then open one up with an image editor, make changes, and see what you get. Of course, your new checkboxes will only work on computers that have the edited images. Those images may also be replaced in an update so save them elsewhere too. Good luck.

I'm not sure which was your question.