Microsoft® SQL Server 2012 Unleashed

Secondary Data Files

A database can have any number of secondary files (in reality, the maximum number of files per database is 32,767, but that should be sufficient for most implementations). You can put a secondary file in the default filegroup or in another filegroup defined for the database. Secondary data files have the file extension ".ndf" by default.

Following are some situations in which the use of secondary database files might be beneficial:

You want to perform a partial backup. A backup can be performed for the entire database or a subset of the database. The subset is specified as a set of files or filegroups. The partial backup feature ...