Australia What does a negative value in an invoice sheet mean?

[limpfisch]

Here is an extract from something I'm working on.
I'm completely new to this kind of thing so forgive me for my basic question, but on the last row you'll notice that the quantity is negative. Am I correct in assuming that this means the product was returned to the seller? If so, how should I treat this when considering the overall sales of the product?
Thanks for the help.

 

[Fidget]

Could be a return, or could be an error. Either way, when considering your overall net sales of the product you don't include it as a sale seeing as you haven't actually sold it.