Due to a change in creating resource names in 2.0, the only layer we
create when signing is "n1". In 1.8, it was n0.
n1 is "very wrong", it is meant to be "Validity layer, used for the
unknown and valid state".
Using n0 was also wrong all the years, it is the background layer. My
understanding after reading the text below is that we should create an
empty n0 layer and put text and image into n2.
https://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PPKAppearances.pdf
The manipulated portions of the signature appearance were contained in
layers n1, n3 and n4. Beginning with version 6, Acrobat does not
maintain support for signature appearances that can be manipulated,
though legacy signatures with these appearances may continue to display
correctly. Use of layers n1, n3, and n4 is not recommended.
The standard XObject layers are as follows:
n0 — Background layer, as preset when creating the signature field, for
example by using the Acrobat
form creation tool. In the example appearances in Example signatures on
page 7, this layer is blank.
n2 — Signature appearance layer, containing information about the
signature. In the example Unverified signature, this is the layer that
contains the line art for the handwritten signature and the text giving
the name, date, reason and location of the signature. The content of
this layer can be dynamically created when the signature is created, but
thereafter remains static. Specifically, it remains static when the
validity state is changed.
