In Advance Steel you can set a tolerance value under which almost identical parts are considered as identical.
The tolerance value is controlled by a default called "Tolerance for identical parts identification" and can be found in Management Tool 
Defaults Numbering General.
This takes into consideration:
- Differences in beam length. If the difference between two beams is smaller or equal to the default value, then the two beams will get the same part mark.
- Difference in width / length for a plate. You can have both width and length difference between two plates, but both values should be smaller or equal to the "Tolerance for identical parts identification" value.
