do not use "on". it is not right. at us the exporter will all smooth out that has identical groups of smoothing. for example, a wing and a pylon can be different objects but have one smoothing group. when exporting, problems may arise.
We use "marked, by default - off".
prescribe "SMOOTH_NORMALS = 1" only on those parts that need to smooth out the normals - for example, a fuselage divided into parts, to prescribe different arguments for damage.
on those that do not participate prescribe "SMOOTH_NORMALS = 0".
https://gyazo.com/7290399688841890c36407e93025cdc6