There are several considerations when choosing sorting options in Secondary Index Plus. The first is whether to go internal or external. The general rule is: Use SIUSORT(I) for internal sorting of 3 or fewer indexes and use SIUSORT(E) for external sorting of 4 or more indexes. This ensures maximum performance with a sort task for each index.
The next thing to consider is sort work space. The best way to do this is to NOT delete and define the indexes prior to rebuilding them. Use the IDCAMS keyword and extract the record count(s) from the ICF catalog, pass it to the sort, and then perform the IDCAMS DELETE/DEFINE. Sort will dynamically allocate the work files using the record counts provided. You could use the FILSZ keyword to pass record counts, but this requires you to monitor the index sizes and adjust them if they change.
We also provide sizing options at a less granular level. Use the SORTWORK keyword in PLUSIN to provide a one-size-fits-all for the number and size of the sort work files. The default is 3 of 100 cylinders each. You can change this in DLIGSET0 using the SORTWORK parm.
All of these options are documented in the MAXM Reorg Solutions for IMS User Guide and you can always call BMC support for help in deciding which is best for your site.