System Integration Testing. It is the environment where Internal testing will take place. Say your testing team's work area.
After SIT is over the app and its respective data are moved to UAT schema, where user Acceptance Testing will be done. Say your clients technical team will test that. Finally after every testing is passed the app is moved to PROD environment.
It is a production environment where live data will be there.
* For developers usually seperate schema called DEV is used for development purposes..
It also applies to every form of software release management and roll-outs, not just to databases. Enterprise QlikView set-ups usually involve multiple separate QlikView platforms as to not endanger the final applications and in order to perfectly measure and control the development and roll-out of new applications and their associated risks.
Another workflow diagram uses similar acronyms: DEV => ACC => PROD, where DEV = DEVelopment, ACC = ACCeptance and PROD = PRODuction.