It is cloud textures.
It works for everyone, whether
they have default or any
addon cloud textures because it
replaces existing cloud textures.
At the moment it is not part of any other
product but REX are saying that it will
become part of REX 4 but will
still have to be paid for.
I too have the REX products you have.
When I run REX Soft Clouds, low level
clouds become soft.
When I run REX Essentials and ask it to replace
cloud textures, they revert to the REX Essentials
versions.
If I run REX Essentials after Soft Clouds and ask it not
to install low level cloud textures, they remain soft.