Normal NFC tags do not work when placed directly onto metal surfaces. Therefore, when using NFC tags on metal you either need a gap of at least 0.5 cm between the tag and the metal or you can use a special tag which has a barrier material, called ferrite, between the NFC tag and the adhesive. This barrier material gives you the same effect as if you had created the gap between the metal surface and the tag.
These NFC tags designed so they can be placed on metal surfaces are called 'on-metal' tags (or sometimes anti-metal).
Note that on-metal tags will also work on non-metal surfaces. However, on-metal tag scan distances always tend to be slightly less than standard NFC tags so unless you need tags that will or might go on metal, you are usually better choosing a standard tag.