What's in the core layer of the €1 and €2 Euro coins?

Hi all – The center disc of €1 and €2 coins is composed of three layers. But the specs are confusing since they just say Ni7 and Ni12 for the core layers. That layer is referred to simply as "nickel", in contrast to nickel-brass and copper-nickel for the two alloys in use.

If it's pure nickel, why would they refer to Ni7 and Ni12? What do those numbers mean in a context where no other metal is named? See below:

Euro coin specs by Sunbird posted Apr 13, 2022 at 12:21 PM