Direct Answer
In Tekla, a 'part' is a single steel member (one rolled section or plate). An 'assembly' is a collection of parts joined in the shop — for example, a beam with welded shear tabs and stiffeners is one assembly. Assembly marks identify what gets shipped from the shop; part marks identify individual pieces within an assembly. Your shipping list should be organized by assembly mark, not part mark.
Full Explanation
The assembly mark (often called the 'mark number' or 'erection mark') is what the ironworker calls out on site. If an assembly is dropped and needs to be re-ordered, you reference the assembly mark. Within an assembly, each part has its own part mark. Some parts are used in multiple assemblies (a standard shear tab, for instance, might be part P-123 appearing in 40 different beam assemblies). BOM reports can be generated at the part level (for material purchasing) or assembly level (for shipping and erection planning).
What This Means for Your Shop
Your receiving department should verify incoming material against part marks (matching the material cert to the specific section). Your shipping department should organize material by assembly marks. Mixing the two causes significant shop floor confusion.
Common Mistakes
Shipping material sorted by part mark when the erection crew needs assemblies. An ironworker calling for 'B-47' needs the entire fabricated assembly — beam plus its tabs and stiffeners — not just the bare beam.
Recommended Process
Organize your shop floor workflow by assembly: all parts for an assembly are staged together, fabricated as a unit, inspected as a unit, marked with the assembly mark, and shipped as a unit. Use the assembly mark as the primary tracking identifier.