step sequencer program

I have just had a conveyor system installed that uses a slc5/03.I have limited programming knowledge but i can read ladder logic for the sake of fault finding.The problem is The programmer has used what looks like step sequencing.I have never seen this type of programming.How does this work and how do i relate it to the process?

MY basic explanation

Anytime a sequence is involved it means that sequence 1 must be fullfilled before sequence 2 can engage....ie

On a conveyor product must be detected and PASS a point before MORE product can move to that area.

The specifics depend on your system, explain it OR show the code and it may be explained better.