Thank you, David,
I'm working with third party seller, they said that we have to buy it. Because, at on-premise side, we're still using other services, or application such us File Server, SCCM,... so for all users is migrated to O365, they still need Core CAL Bridge to work with those applications at on-premise.
(My company has 300 users. We will only move 100 users to O365 to use email, team,....)