I didn´t know exactly what you have done.

Field is type number without decimal places in my form.

The checkbox in my xpage has value if checked = @True();  value if not = @False();

If I save this xpage it will return "1" or "0" as string; but without any error.

When I saw this I too thought this was for storing Boolean values, but the problem is there is NO underlying document field type that supports storing a Boolean Value.

so this is a converter but then you still have to convert the value back to a integer for storing it.  No really very many use cases for this AT ALL in my opinion.

I mis-read the default of the check box when it says the Default Checked return is true it does not mean 1 but the literal string "true" So after a lot wasted time figured out that needed to add a convert .

getAsString value == 1 ? "true" : "false"

getAsObject value == "true" ? 1 : 0

So the saved value is boolean  1 or 0

The solution is pretty simple but not at all obvious. The documentation gives very little help