I am used to strength being measured in units of stress, such as pounds per square inch, or Pascals.
I am assuming what the author is trying to say is, "this filament will break when holding 18 kg," is that correct?
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Material strength is generally expressed in the form of stress but for a component using a load is perfectly logical. Equally while using Newtons would be more precise quoting an ultimate load as a mass is often more practically useful.
What is less clear is whether this is a 'safe working load' (ie you can hang a mass of 18kg off it and it won't break) or if it is the ultimate load.
Safe working loads generally include a factor of safety to account for dynamic loading and quoting the load in kg rather than N sort of implies this but assuming 18kgf as the breaking load would be more conservative in the absence of any other information.