Hi there,
May I know what you mean by 'customize'? Perhaps you can elaborate more on what exactly you want to customize for cluster categories so we can advice you accordingly, thanks.
In the backend, the cluster categories are located under the cluster item themselves(for example group categories: http://take.ms/ni4QL).

Got it, thanks Raymond.

Hey May,

You are most welcome.

As a gentle reminder, kindly start a new thread if you have any other issue in the future so it will be easier for us to manage your inquiry. I will lock and mark this thread as resolved.