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In Article tab, for cable use quantity "0", because we do not want quantity but length. FOTOBazy artykułów Helukabel:

don`t trow away the quantity (you may think that this its not used - not true)
you should multiply the length by quantity then the result its correct

why ?

because quantity show how many "pieces" of cable witch this length its used
example :
if you have incoming supply witch multiple cable like this 3x(3x120mm2) - one piece 100m
then if your don`t include the quantity
you will become only 100m but it should be 300m from cable 3x120mm2 in this example

and second taking into account the quantity you can filter out all articles that have "0" quantity and don't show it
that give you some capabilities to hide certain articles in forms and use templates that you have in database in this articles