If engineered timber is to be floated, then a 2mm dampscreen underlay should be installed. I have not heard of engineered flooring being nailed down, but if you are wanting to "fix" to the subfloor, then you will need a moisture barrier over the sub floor before it is glued - (generally how we do things here in NZ).
The timber attracts moisture, so with either a moisture barrier or dampscreen underlay, the intention is to prevent the moisture from penetrating the timber.
Depending on which product you are looking at, there is plenty of information on their websites which will help.
Flooring Vision
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