"v3" are the new clouds introduced in Terragen 4. I know that can be a little confusing.
"v2" were the old clouds in Terragen 2 an 3.
In the Atmosphere Layout, when adding a cloud layer, if you use the "Generic" presets you get a general v3 cloud, with the same capabilities as the v2 clouds (e.g. all the external shader inputs, control of noise function, etc.). The others are presets, mostly using Easy Cloud (a variant of v3 clouds), except the Global Cloud Layers which use v2 cloud model because v3 has less benefit from orbit (and also is limited to local clouds).
As others said, if you just want to create a v2 generic cloud you can do so in the node network by right-clicking to add a node.
- Oshyan