Signature Dictionary 'Filter'

I have a question about the 'filter' in the signature dictionary. It is defined as (taken from PDF 1.4 due to PDF/A):

Filter name (Required; inheritable) The name of the signature handler to be used for authenticating the field’s contents, such as Adobe.PPKLite, Entrust.PPKEF, CICI.SignIt, or VeriSign.PPKVS.

1.) A plugin is used to validate signatures, right? (please correct me if i am wrong here)

2.) Can the subfilter (like adbe.pkcs7.detached or adbe.pkcs7.sha1) also be used with other filters than adobe.ppklite?

2) The Adobe.PPKLite is the "default" plugin? (which every reader since 5.0 or so has)

3.) The other handlers are also part of acrobat? (such as entrust, verisign etc.)

4.) What must i do to register my own handler?

5.) What would happen if one would set the filter which is not existent?

Thanks for clarification,

ToM