Direct Answer
Tekla numbering settings define how piece marks are assigned — the prefix system, counter increments, and which attributes differentiate one piece from another (e.g., whether mirrored members get unique marks). Getting numbering settings wrong before locking the model can result in duplicate piece marks for non-identical members or separate marks for identical members — both are costly problems.
Full Explanation
In Tekla, numbering is controlled by numbering settings files (.num) that define: prefix conventions by member type (B for beam, C for column), position number ranges, what constitutes a 'unique' part (material, profile, length, rotation, or assembly). A mirrored beam is geometrically different from the original — whether to give it a unique mark depends on whether the mirror creates a different fabrication sequence. Running numbering too early (before all connections are modeled) or changing settings mid-project can cause piece mark conflicts.
What This Means for Your Shop
Piece mark conflicts — two different members with the same mark — are among the most disruptive detailing errors in fabrication. If two differently-coped beams share a mark, your shop will fabricate the wrong member for one location. Confirm with your detailer that numbering was run after all connections were finalized.
Common Mistakes
Running Tekla numbering on a partially modeled project, then adding members that collide with existing numbers. Also: changing numbering settings after drawings have been issued, which scrambles existing piece mark assignments.
Recommended Process
Lock numbering settings before running numbering for the first time. Only run numbering after all members and connections are modeled. If numbering must be re-run after drawings are issued, prepare a renumbering log comparing old and new piece marks.