It is labelled “Sodium hypochlorite ± 3,5 % active” and I thought it’s a 3,5% solution of NaOCl, but I guess the 3,5% means something else? What exactly and can I use it for decontamination?
It is labeled "Sodium hypochlorite ± 3,5 % active Cl" The active Cl is the disinfecting part of the NaOCl solution. This is what you nedd, and it appears that what you bought is slightly less concentrated that what is recommended in the protocol you linked to. But I guess you could just go to the nearest shopping mall and buy a bottle of no name thin bleach (hypochlorite based of course) should have the percentage of active chlorine somewhere on the label.