Many legal systems also apply a tiered approach to evidentiary weight. A simple electronic signature cannot be denied legal effect merely because it is electronic, but its probative value is left to the judge's discretion. An advanced or certified signature, by contrast, enjoys a strong presumption of validity, shifting the burden of proof to the party who challenges it. This weight nonetheless faces practical difficulties. The person to whom a signature is attributed may deny it or allege forgery, in which case the party relying on it must prove that its conditions were met, usually with the help of technical experts. Some legislation also excludes certain acts, such as real estate transactions and documents that the law requires to be in official form. In addition, the security of the technical system and the preservation of electronic records are critical, and cross-border contracts raise questions of governing law and mutual recognition of certification authorities.