The global datasource user is the account used by the system to provide administrative access to data sources on the platform. The account should be able to access all data sources through standard "Windows Authentication" which may or may not need to work across multiple domains.
NOTE: If the system is using Windows Authentication for communicating with SQL Server, the global datasource user account needs appropriate privileges.
In contrast to the global datasource user, it is also possible to define a server-specific datasource user. When you define a server-specific datasource user, the global datasource user is no longer able to access that specific server.
For organizations taking a centralized approach to data administration, having a single global datasource user usually makes sense. But for organizations taking a more distributed approach to data administration, it may be desirable to assign a separate datasource user for each specific server.