When we create a datasource in SSAS, we need two types of connection information
As the name goes, this information is used to connect to data source. This can be a standard User ID / Password combination (can be used for SQL Server, Oracle or any DB). If you connect to SQL Server, you can use Windows Authentication also (preferred method due to better security) for this.
This specifies a Windows account used for SSAS Server management such as cube processing (create aggregations, SSAS indexes and write those into SSAS internal files such as *.lstore, *.asstore found in D:\Program Files\Microsoft SQL Server\MSAS10.SQL2008\OLAP\Data\<ProjectName>\ ). This cannot be a SQL account since OS tasks need to be achieved using this authentication.