There are 4 events, that are fired by the API during the execution of the AddSignature function. All of them must be handled by the client application or the API will not be able to add a signature to a document. If you want to work with documents in read-only mode, e.g. check signatures or decrypt biodata, but don't want to add new signatures, then it's not necessary to provide event handlers.
Only the OnLCDSignButton Event is optional, for signing a pdf document the eventsOnGetAesKey, OnGetDeviceCertificate, OnGetSignedDocHash and OnReadHighResBitmap are mandatory !