Files and filegroups, however, help to easily add new files to new disks. Additionally, if your database exceeds the maximum size for a single Windows NT file, you can use secondary data files to grow your database further. For example, the files sales.mdf and sales.ndf, which contain data and objects from the sales database, cannot be used by any other database. A file can be a member of only one filegroup. Data and transaction log information cannot be part of the same file or filegroup.
Transaction log files are never part of a filegroup.
A database comprises a primary filegroup and any user-defined filegroups. The filegroup that contains the primary file is the primary filegroup. When a database is created, the primary filegroup contains the primary data file and any other files that are not put into another filegroup. All system tables are allocated in the primary filegroup. If the primary filegroup runs out of space, no new catalog information can be added to the system tables.