About user-defined file systems and availability
Normally, all disk units attached to a system are treated as a single logical disk unit. The operating system places data across the disk units to optimize performance and disk utilization. However, an i5/OS administrator has the option to create logical groupings of disk units, called auxiliary storage pools or ASPs. Auxiliary storage pools provide several possible benefits, for example:
If you want to place your Domino data directory in a user ASP, you need to create a user-defined file system (UDFS) to map the integrated file system directory to the correct ASP.

You can read more about ASPs and creating user-defined file systems in the Backup and Recovery book (SC41-5304-06).