The Derived Column Selection icon indicates that a column in the row selected had to be mapped or derived before selection criteria could be applied against the row. For example, given FLD1=:HV1 where FLD1 is defined as CHAR(6) and the value for :HV1 is 'AAAAAAAAAA', column FLD1 must be mapped from a CHAR(6) to a CHAR(10) before it can be compared to the value 'AAAAAAAAAA'.
Derived column selection is the slowest selection method. If a column cannot be performed by another method, it will be performed as derived selection. The column may be derived for many reasons, for example: