Hello. So SWIM lives in Canada and wanted to find lye. He found that home hardware sold it and was happy to hear that. SWIM located the safety data sheet and could not understand what was written there. He found composition and under that heading was only listed sodium hydroxide. Sodium Hydroxide was followed by the cas number and then followed by concentration (% w/w) which was listed as 60-100. SWIM does not want to use this product if it is not pure sodium hydroxide. Especially not if this number means it’s possible that it is only 60% sodium hydroxide. Is this what concentration (% w/w) 60-100 means? The label clearly says 100% pure lye crystals but the data sheet is misleading. SWIM has read a man’s post on another site that says he contacted the manufacturer and they said it was pure sodium hydroxide. But SWIM would like to hear from someone experienced with safety data sheets to explain what this concentration (% w/w) means and if he should be concerned. SWIM appreciates any knowledge in the subject and thanks you in advance. SWIM just wants to be safe.

If it didnt even give a hint of a second ingredient, the jar says pure lye, and the customer service says only sodium hydroxide then the most likely scenario is that they are covering their ass just in case some water or CO2 makes it into a batch of lye.
Lye sucks up water like a sponge and CO2 will convert it into sodium carbonate, so those are the primary impurities of 'pure lye'. A small amount of either wouldnt be a problem for most applications discussed here.