The source data set must be a base VSAM KSDS, not an alternate index. However, updates made to the KSDS via an alternate index are reflected in a CICS®-maintained data table.
The VSAM definition of the KSDS supplies the values for maximum record length and key length.
For a user-maintained data table, the updates are not reflected in either the source data set or its alternate indexes. A user-maintained data table is entirely independent of its source data set after loading has completed.