Attic, which flooring to install?

What is this -1 supposed to mean?

You otherwise run into the press, which causes problems, so plan for 1 cm of clearance. We installed quite a high battens beneath and then OSB on top. In addition, we have a circumference clearance of 1 meter to avoid sealing the attic. However, when building the roof truss, we also had the material brought up immediately.

Both our timber specialist retailer and the building materials dealer have said that tongue and groove boards are waste and there are no quality classifications.