A typical installation consists of a number of distinct applications that make up a service. Each service usually has a number of "Terminal-Owning Region" (TORs) that route transactions to multiple "Application-Owning Regions" (AORs), though other topologies are possible. For example, the AORs might not perform File I/O. Instead there would be a "File-Owning Region" (FOR) that performed the File I/O on behalf of transactions in the AOR given that, at the time, a VSAM file could only support recoverable write access from one address space at a time.