What is master secret?

As I was studying the Indy processor, I was curious about the Master Secret and asked a question. When writing a credential definition in the ledger and checking the value written directly in the ledger, I was able to check the information about the master secret in the primary attribute.
There are two questions.
One.
The indy document states that the master secret is created by holder (alice), but the master_secret created by holder is created using the master_sercret written in the ledger.
2. Then, I wonder if multiple holders can make master_secret using the master secret value of the credential definition written in the ledger.