another golden oldie from the bucket full of dusty emails sitting in the corner... hopefully useful to somebody:

IBM Support Line Technical Document

Document Title: Specifying ASP When Using IFS

Document Description:
IFS commands such as MKDIR and IFS APIs do not provide an option to specify which ASP to use. This is because UNIX/POSIX APIs, on which IFS APIs are based, do not have a concept of ASP. The following steps show how you can use a particular ASP when using IFS APIs or commands on an AS/400e or iSeries 400 system.

To determine if you have multiple ASP on the AS/400e or iSeries 400 system, on the OS/400 command line type the following:

WRKLNK OBJ('\DEV')

Press the Enter key. Select Option 5 to move to the Next Level.

If your system has only one ASP, you will see only QASP01. Otherwise, you will see additional QASPnn links.
You can have as many as 98 user ASPs plus one system ASP on the AS/400e or iSeries 400 system.

Before you can use a particular ASP from IFS, an User-Defined File System (UDFS) must be created in that
ASP. To create the UDFS on the OS/400 command line, type the following:

CRTUDFS UDFS('/DEV/QASP02/ASP02.UDFS') DTAAUT(*RWX) +
OBJAUT(*ALL) CASE(*MONO)

Notes:
1       Run the above command one time only.   
2       To have the file system case-sensitive, select *MIXED; however this is not recommended. This command creates an User-Defined File system called ASP02 in ASP02.
3       Before using this file system it must be mounted using the Mount command. Mount is not preserved across IPLs. Therefore, you must mount after every IPL. It is recommended that you place the Mount command in the start-up program for the system.    
To mount the User-Defined File System on a object-link '\M1', type the following on the OS/400 command line:

MOUNT TYPE(*UDFS) MFS('/DEV/QASP02/ASP02.UDFS') MNTOVRDIR('/M1')

To create Object-link M1, type the following:

MKDIR('\M1')

It is recommended that you do not use Object-Link '\M1' when the file system is not mounted. If you created Object Links under '\M1', those are no longer available when the UDFS is mounted over this directory.

Once the UDFS is mounted you can create new links that will be stored in ASP02. For example,
the following command creates a directory in ASP02, and the second command copies a CL source file there:

MKDIR OBJ('/M1/ALAM')

CPYTOSTMF FROMMBR('/QSYS.LIB/QGPL.LIB/QCLSRC.FILE/QSTRUP.MBR') +
TOSTMF('/M1/ALAM/STRUP.TXT') DBFCCSID(037) STMFCODPAG(437)

The file system is unmounted during IPL or you can use the following command when you are done:

UNMOUNT TYPE(*UDFS) MFS('/dev/QASP02/ASP02.UDFS')