Unlike the mainframe, a VSAM KSDS or ESDS data set is usually represented under Windows or UNIX by two files - a data file, which contains the actual data records for the VSAM data set, and an index file, which contains the primary and alternate key data. These files usually have filename extensions of .dat and .idx respectively. The exception is where IDXFORMAT8 files are in use; these have just a .dat file.
Although the two-file structure affects the way that IDCAMS commands are processed, it does not change the final outcome of the commands. The results of executing commands in a non-mainframe environment is the same as on the mainframe. MFJAMS ignores those parts of a command that have no relevance in a non-mainframe environment. For example, because the VSAM file structure under Windows or UNIX does not provide CI or CA sizes to manipulate, these command parameters are ignored.