Offences Relating to Documents
Making a false document/electronic record means: (A) dishonestly/fraudulently making, signing, sealing, executing, transmitting, or e-signing a document, intending it believed made by someone who didn't actually authorise it; (B) dishonestly/fraudulently, without lawful authority, materially altering an already-made/executed/e-signed document; or (C) dishonestly/fraudulently causing someone to sign/execute/alter a document (or affix an e-signature) while they can't understand its contents or nature due to unsoundness of mind, intoxication, or deception. A person's own signature can amount to forgery (e.g. signing your own name intending it to be believed a different, same-named person signed); fictitious or deceased people's names count too.
This is the foundational definition of document forgery for the chapter that follows - notably broad, since it covers not just faking someone else's signature but also materially altering a genuine document afterward, tricking an incapacitated person into signing something they don't understand, and even signing your OWN name in a way intended to deceive.
Altering a genuine cheque after it was signed - for example, adding extra digits to inflate the amount - meets this section's definition of making a false document, exactly as much as forging someone else's signature from scratch.