what are utilities, if not programs?
Here are some pointers to helpful documentation:
Here and here
What are the advantages of using this?
One of the advantages is that you don't have to write a program for each table you need to access.
Just write the SQL command and run it!
In the below two cases when do I use PLAN as DSNTIA91 and when do I use plan as DSNTIAD?
You must have 2 different versions of DB2 installed.
DSNTIA91 would run with DB2 v9.1 and DSNTIAD with an older or newer version.
Check with your administrators. Definitely not standard naming convention.