What counts as a part here
The selection list contains resolved mesh instances reached from printable build items. One reusable mesh can appear several times with different component or build transforms; each occurrence receives its own numbered STL. An assembly containing two leaf meshes therefore appears as two selectable parts, even when the source slicer shows one top-level object.
Nested transforms are applied from the leaf mesh outward to the build item. Translations, rotations and supported scaling affect the exported coordinates. A reflection reverses the triangle vertex order so that mirroring does not accidentally reverse the source surface orientation.
Select and export a useful subset
Run the conversion once to see the numbered instance table and preview. Leave the form’s part field blank for everything, or rerun with a comma-separated list such as 1, 3. The preview checkboxes offer a second convenient selection: their own download buttons export the currently checked instances. The main report and its original download buttons retain the selection used for that conversion.
The combined STL contains all chosen triangles in one file. The parts ZIP contains one binary STL per selected instance plus manifest.json and a short README. Part filenames use stable numbers, while the manifest records the source object name, source model path, build item, transform and millimeter bounds.
Preserve placement when reopening
Individual STLs retain the source build coordinates; they are not automatically centered at the origin. Import them together with an option that preserves placement or treats them as an assembly. Some slicers automatically center each imported file, which can destroy the relationship between separate pieces even though the STL coordinates are correct.
A combined export is not a Boolean union. Intersecting solids remain intersecting triangle surfaces, and disconnected components remain disconnected. The converter does not infer a single watertight printable object. Check the receiving slicer’s preview, intended plate layout and support settings before generating machine instructions.