SignRequest uses 'SHA-256' cryptographic hash functions (hash codes, for short) to ensure the security of our electronically signed documents.
A cryptographic hash function is a mathematical algorithm that maps data of arbitrary size (often called the "message") to a bit string of a fixed size (the "hash value", "hash", or "message digest") and is a one-way function, that is, a function which is practically infeasible to invert.
Hash codes are unique codes specific to an individual document. This is a great way to ensure that the document you are looking at is the original. Even if 1 pixel is changed in the document, this will result in a different code. The document that generates the hash code, that is visible in the SignRequest email and the signing log, will be the original document.