File units: File units are the measurement units, such as millimetres or inches, in which a CAD file's numbers should be read; some formats like STL store none, so a part can arrive at the wrong size.
A 3D file is a list of numbers. Whether “25.4” means 25.4 millimetres or 25.4 inches depends on the units, and not every format records them. STEP and 3MF do. STL and OBJ do not. DXF can, but often the field is left blank.
When units are missing, software assumes. Most 3D printing tools assume millimetres. A part designed in inches then arrives 25.4 times too small in each direction. A part in metres can arrive 1,000 times too small, or a millimetre part read as inches arrives 25.4 times too big.
Example: a buyer designs a bracket 4 inches long and exports an STL. The quoting tool reads it as 4 mm long. The volume, and so the price, comes out about 16,000 times too low (25.4 cubed). If nobody notices, the shop either ships a tiny part or loses the job when it re-quotes.
The fix is to ask. A sensible check compares the size with what is likely: a 4 mm bracket with 0.1 mm holes is suspicious; a 100 mm one is not. The blog post how file units cost you money covers the cost side.
Quotometry asks the buyer to confirm the units when an STL arrives, because the file cannot say. Read STL vs STEP vs 3MF for which formats carry units.
Go deeper: STL vs STEP vs 3MF (and OBJ, DXF, IGES): which file to send for a quote