If you edit the file, the signature will not be valid anymore.
If you remove your console id, so that the cia will install on other people's 3DSs, the signature is not valid.
If you patch the demo play count so you can remove the 5, 10, 30 play limit from the demo, the signature is not valid.
If you patch the dlc permissions to set that you have permission for all the dlc, the signature will not be valid.
If you decrypt the contents and edit the game, make it region free, or translate the game it etc, and then re-encrypt, the signature will not be valid.
Only nintendo can create these signatures.