A SRN is a unique number that is allocated to you by the publicly listed company that issued the shares. Unlike a Holder Identification Number (HIN) you will have a different SRN for each listed company in which you hold issuer-sponsored shares.
These shares are held on the issuer-sponsored subregister maintained by the listed company that issued the shares. Most publicly listed companies use a share registry to administer the issuer-sponsored subregister on their behalf. As a result, most issuer-sponsored holdings held in a SRN account are held by a share registry.
You can find your SRN on an Issuer-sponsored Holding Statement. A SRN starts with the letter I and is followed by ten numbers, for example I0001234567.
The share registry your shares are held with will send you a holding statement with your SRN listed. If you cannot find a holding statement, contact your share registry to ask for one.
If you are unsure of which registry, you can find it in the details for the listed company.