To calculate the number of buffers you need, you must determine the number of strings you will use. A string is a request to a VSAM data set requiring data set positioning. If different concurrent accesses to the same data set are necessary, multiple strings are used. If multiple strings are used, each string requires exclusive control of an index I/O buffer. Therefore, the value specified for the STRNO parameter (in the ACB or GENCB macro, or AMP parameter) is the minimum number of index I/O buffers required when requests that require concurrent positioning are issued.