Can you please show how to properly use the SELECT OPTIMIZED command ?

Here is a sample input for the SELECT OPTIMIZED
...
PARAMETER 1
CODE AISC UNIFIED 2005
METHOD ASD
FIXED GROUP
GROUP MEMB 1 TO 10
GROUP MEMB 11 TO 20
GROUP MEMB ...
SELECT OPTIMIZED
PERFORM ANALYSIS
CHECK CODE ALL

The SELECT OPTIMIZED command actually is equivalent to the following  internal steps

CHECK CODE ALL
SELECT ALL
PERFORM ANALYSIS
SELECT ALL

The FIXED GROUP specified ahead of the SELECT OPTIMIZED ensures that the sections are grouped after the final SELECT ALL command ( processed internally as part of the SELECT OPTIMIZED command ). The PERFORM ANALYSIS after SELECT OPTIMZED ensures that the structure is re-analyzed based on the sections after the grouping which would result in redistribution of member forces. The final CHECK CODE checks whether the final sections are able to satisfy the code check based on the redistributed member forces.