If the MFS needs to update a file other than the current file, it is best to use BASIC+ file I/O commands (OPEN, READ, WRITE, DELETE) rather than a direct call. If a direct call is used, the MFS will pass a call to the next filing system (MFS or BFS) for the current file. This may cause problems in two ways.
First, the MFS that follows the current MFS may be installed for only the current file, and not for other files. A direct call might thus inadvertently "install" the next MFS onto the second file being accessed.
Another problem is the converse of the first. By doing a direct call using the MFS list for the current file, an MFS may bypass the MFS list for the second file. For example, if an MFS uses a direct call to access a file that is indexed, SI.MFS will not be called unless by chance it appears in the BFS argument for the current file.
A corollary to this is that the second file may use an entirely different BFS. Obviously, a direct call from the MFS can only use the BFS for the current file.
Some applications might present a hybrid of these problems. An MFS might use BASIC+ commands to access a second file, only to discover that the second file also uses the current MFS – thereby resulting in a call to itself.