SRN stands for Securityholder Reference Number. An SRN is used to identify the owner of the shares that are held by a share registry (i.e. Issuer Sponsored Shares).
Shareholders can find out how their shares are held from the reference number on the top right corner of their latest holding or dividend statement. If the reference number begins with an "I" or "C" and is followed by nine to eleven numbers, this is a Shareholder Reference Number (SRN) which means the shares are Issuer Sponsored. There are unique SRN numbers for each Issuer Sponsored holding.