I have been using the interceptAction in a Form to do 'stuff' when the
user clicks on Submit.

The docs on page:

For this function explain:

InterceptAction(actionname, order, function)

form.interceptAction("actionname", "order", "function")

Allows you to intercept the script attached to an action button. The
function passed in is executed based on the order parameter.

Valid actionname values are SubmitAction and CancelAction.

The choices for actionname for an approval form are: ApprovalAction,
RefusalAction, DenyAction, UpdateAction, CancelAction and
CommentAction.Valid values for the advice parameter are:Before: The
function is called before the script attached to the button
executes.after: The function is called after the script attached to the
button executes.around: The function is passed a parameter that allows
you to decide whether to execute the script attached to the button The
following example shows the submit action intercepted. The form is only
submitted if the user replies Yes.

window.inv=function (invocation) { if (confirm( "Are you sure you want
to submit?")) { var result = invocation.proceed(); return result; }; };

form.interceptAction("SubmitAction", "around", window.inv);

Ok, I have a working example, but I have many questions remaining.
Which is my usual issue with the docs. I need better and more examples.
All values and options need to be listed in the docs. After all, where
else do you go, to find out such things?

1) invocation.proceed() makes sense, and allows the Submit to proceed.
What other alternate functions are there? Can I fail to proceed? If
so, what is the format for that?

2) Is there a way to take a click on Submit, (intercept the action) and
then if the test fails, then cancel the entire form? What, other than
proceeding, can I do with this function? What happens if I do not send
a .proceed()?