what is certification level?

[Antonis Tsolomitis]

Is there any easy documentation of what is the "certification level"?
I ask because if CL is set to anything other than NOT_CERTIFIED
Acrobat (as well as other systems) refuses to add a signature.

But with jsignpdf I can sign again (add signature) a file
whose 1st signature set the CL to other values too.

So what is going on with Certification Level?

thank you,

Antonis.