Joined: 23 Nov 2006 Posts: 19270 Location: Inside the Matrix
The really short version is that to do this, you issue commands not usually associated with standard application code.
If you search in the forums, you will find multiple topics about dynamic file allocation.
It is very rarely a good choice. It is another "feature" that while it may be done, there is usually no business reason to do it that way. It might be fun for a programmer to figure out how, but it will not usually be in the interest of any user (which is why most of us are working).