what is difference between read unique and clean unique?

In ace spec, before master writing cache , it needs do read unique ,clean unique, make unique.

Q1:I am confused with them. Why we need three operations instead of just one operation ? 

Q2:In clean unique,  another cache in dirty state in other master needs to be written to memory. Why make unique does not need it?

Please explain more about three operations

Thanks a lot !!