Define the board reference
Top and bottom identify opposite sides in the CAD and manufacturing definition. They do not necessarily describe how the finished product sits in its enclosure. Either side can contain components, test pads and routed copper.
State the viewing convention in drawings and keep a recognizable datum or asymmetric feature available for orientation. A symmetric board can conceal a mirrored or rotated interpretation.
Separate viewing from manufacturing geometry
A CAD viewer may show the bottom through the board or as a flipped physical view. Those display modes should not be confused with instructions to mirror manufacturing data. Use the export convention agreed with the recipient.
Check legend readability, connector orientation and hole alignment in the exported package. Do not manually mirror files simply because a bottom-side image appears reversed in one viewing mode.
Verify bottom-side placement
Placement files require an explicit origin, units, side and rotation convention. Compare several known polarized or asymmetric parts against the assembly drawing and footprint. Different tools or recipients may interpret rotation differently.
Review bottom-side component height, support, reflow sequence and test access with the assembler. Components near edges or fixtures can limit handling even when they fit within the electrical outline.
Use a coherent release and first article
Keep BOM references, drawings and coordinates tied to the same board revision and variant. Clarify single-board versus panel coordinates. Include special orientation instructions where a footprint alone is insufficient.
Inspect a representative first article for side, rotation and polarity before repeat assembly. Record any agreed coordinate conversion so future builds do not rely on an undocumented correction.