For a custom component enquiry, send the model format the supplier can work with and a drawing that states the requirements. STEP and STL are not interchangeable descriptions. A file that opens on screen can still be missing the detail needed for a manufacturing review.
A mesh and a CAD exchange file answer different needs
STL represents a surface as triangular facets. The Library of Congress format description notes that it does not carry ordinary colour or texture information, and the format does not define a physical unit. STEP is used for product-data exchange under ISO 10303; the actual content depends on the application protocol and exporter. The file extension alone does not guarantee a complete, editable solid.
Curved surfaces need particular attention
A coarse mesh can display a faceted approximation of a curve. Increasing visual smoothness in a viewer does not necessarily change the underlying geometry. If a supplied mesh is the only available source, tell the supplier what it represents and which features need reconstruction or confirmation. Do not silently treat a scan or display model as the approved manufacturing definition.
The drawing carries the decisions
Include units, critical dimensions, tolerances, material or construction requirements and the revision. State which document takes precedence if the model and drawing disagree. A screen capture is useful for highlighting a question, but it should not become the only source of an important dimension. Keep the original design file alongside the exported exchange file in the project archive.
Check the export from the recipient’s side
Confirm that the supplier can open the selected file and identify the intended component. A quick agreed dimension can help reveal a unit or scale mismatch. Request confirmation of any missing bodies, surfaces or references before quotation details are frozen. That small check is usually cheaper than discovering that the two parties reviewed different geometry.
Technical background: Library of Congress: STL format, Library of Congress: STEP exchange format.