A digital signature is the mechanism for indicating one's approval of DGN files to other users and communicating approval in a verifiable manner. A digital signature captures the current state of a model or file.
A digital signature (not to be confused with a digital certificate) is an electronic signature that can be used to authenticate the identity of the sender of a message or the signer of a document, and possibly to ensure that the original content of the message or document that has been sent is unchanged.
Digitally signing a model or file can be used to indicate approval of the design. Another user can then detect the signature, verify the signer´s identity, and verify that the design has not been changed since it was signed. Model-specific signatures are displayed graphically in the context of the design. Multiple signatures can be affixed to a single file or model. Hierarchical signing is also supported so that one signature is dependent upon prior signatures. A signature can cover the content of a model or file and all of its references including DGN and DWG references.
A signature cell is really just a cell. It is created and managed as a normal cell in a cell library. It is then placed in the model that is being signed. What makes a signature cell special is that it itself is digitally signed
The Place Signature Cell tool is used to place a model signature that applies to and is displayed in the context of the current model. A model signature requires a signature cell that is placed in the current model to represent the signature visually. A model signature applies to the contents of the current model
A red seal next to the signature in the model indicates that it is a signature. Moving or modifying the signature invalidates the signature. An invalid signature is displayed with a red X across it. A yellow warning symbol is displayed if the signature is based on a certificate that is not trusted or if the signature has expired.
Use the Add Signature tool on the Digital Signatures dialog to add additional signatures to a file or model. Adding a new signature does not invalidate existing signatures. The same user can place multiple instances of the same signature cell or multiple signature cells. The same user can also place a mixture of file, model, and hidden model signatures.
To create a hierarchy, the signer selects one or more existing signatures in the Digital Signatures dialog before activating the Add Model or File Signature tool. The signer then specifies that the new signature will depend on the selected signatures.
Digital Signatures - Security - Microstation