Signature field (PDF 1.3) is a form field that contains a digital signature (see 12.8, "Digital Signatures"). The field
dictionary representing a signature field may contain the additional entries listed in Table 232, as well as the
standard entries described in Table 220. The field type (FT) shall be Sig, and the field value (V), if present, shall
be a signature dictionary containing the signature and specifying various attributes of the signature field
(see Table 252).
NOTE 1This signature form field serves two primary purposes. The first is to define the form field that will provide
the visual signing properties for display but it also may hold information needed later when the actual signing takes
place, such as the signature technology to use. This carries information from the author of the document to the
software that later does the signing.
NOTE 2Filling in (signing) the signature field entails updating at least the V entry and usually also the AP entry of
the associated widget annotation. Exporting a signature field typically exports the T, V, and AP entries.
Like any other field, a signature field may be described by a widget annotation dictionary containing entries pertaining
to an annotation as well as a field (see 12.5.6.19, "Widget Annotations"). The annotation rectangle (Rect) in such a
dictionary shall give the position of the field on its page. Signature fields that are not intended to be visible shall
have an annotation rectangle that has zero height and width. Conforming readers shall treat such signatures as not
visible. Conforming readers shall also treat signatures as not visible if either the Hidden bit or the NoView bit of
the F entry is true. The F entry is described in Table 164, and annotation flags are described in Table 165.