What is meant by GLOBAL CHANGE in Primavera P6?

Global change is a command that can be used to make major changes in the underlying database in primavera.
It's a change that can't be undone.
It's used when u want to make a major change on the entire program, which affects all activities or a list of activities that can be filtered out using a query.
It's an external command that gives direct access to the oracle database .

Global change helps us in to make changes for in of activities and their attributes (resource assignments, project expenses).
It helps in saving time from scheduling or doing repetitive change one by one in different activities.
H.S.Nazim Ali

Oracle Primavera P6 Global Change tool enables to modify some significant features and components of the activities. You can read the article to learn more

Understanding the Primavera P6 Global Change