Well, As mentioned, for the client notes. I already have the case notes for the application I was having with them. On their website there is no way you can ask for GCMS client notes. It has to be related to some application (Imm, PR Card, Citizenship, etc.).
Client notes would contain information and notes that were made on your overall client file in GCMS. My case notes for my recent application(client details section) indicates for example that my client file had been updated on a date probably due to interaction via CSE, but of I cannot see what was updated in the case notes.