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The unified atomic mass unit and dalton are names for the same mass unit:where the carbon-12 chemical atom is free, at rest, and in its nuclear and electronic ground state. Its conversion to SI mass units is determined experimentally. See The International System of Units (SI), ninth edition, SI Brochure (Table 8, footnote (f), printed p. 146: dalton and unified atomic mass unit).

The molecular weight cut-off rates retention of reference molecules. One manufacturer's definition uses the mass at which approximately $90\%$ remains after seventeen-hour dialysis, tested with globular proteins. A $10\mathrm{kDa}$ rating uses dalton units: a mass, not a length or an absolute passage threshold.^[ref]

A semipermeable membrane has a distribution of passage sizes, and a molecule's shape affects passage as well as its mass. Near the nominal MWCO, slow passage can make substantial retention compatible with eventual leakage. The protein-retention experiment below makes retention an observable quantity rather than a universal yes-or-no property inferred from the label.