Required non "Off" appearance for checkbox form fields?

[Lonzak]

Until now I had the following understanding of a checkbox form field:

A checkbox must have at least one (non-off) appearance defined and if there is a second optional "off" appearance it must be stored under the name "Off". Now I got the following PDF with a checkbox like this:

129 0 obj
<<
/Type /Annot /Subtype /Widget /FT /Btn
/AP 531 0 R
/AS /Off /V /Off /DV /Off
/F 4 /Ff 0
/H /P
/MK 530 0 R /P 20 0 R /Rect [ 504 497 513 506 ] /T (checkbox1) /TU (checkbox1)
>>
endobj


531 0 obj
<<
/N 834 0 R
>>
endobj

834 0 obj
<<
/Off 1036 0 R
>>
endobj

It fulfills the following paragraph of the spec:

The appearance for the off state is optional but, if present, shall be stored in the appearance dictionary under the name Off.

And it theoretically also fulfills the PDF 2.0 definition for the AP key (seems to have been changed from optional to required):

Every annotation (including those whose Subtype value is Widget, as used for form fields), [...], shall have at least one appearance dictionary.

Sure, the checkbox above has one appearance dictionary defined .... which is the optional "Off". However in my understanding this was not meant by the spec. If my understanding is wrong what should a compliant reader do when such a checkbox is checked? Create a new on-appearance? But what should it be called then (On, Yes, ...)? And how should it look like (cross, checkmark, etc).

So how does the community sees this?

[petervwyatt]

And it theoretically also fulfills the PDF 2.0 definition for the AP key (seems to have been changed from optional to required)

Correct. PDF 2.0 now mandates AP appearance streams (like PDF/A) to avoid implementation dependency when missing.

[Lonzak]

Correct. PDF 2.0 now mandates AP appearance streams (like PDF/A) to avoid implementation dependency when missing.

Interesting. I didn't see that appearance streams are necessary. But either way the above mentioned checkbox also doesn't have an appearance stream... (And it is a PDF 1.4 document).