Please tell us what maker of SoftPLC you intend to use; Maj. Toxido's answer still is valid.
Each PLC manufacturer has a slightly different implementation of IEC-6113-3 standarts. So again tell us what you plan to use, and we can give you an answer.
Or surf Siemens' webpages looking for STL programming manuals; do the same for Allen-Bradley's IL...and so on and so on...