The 1st & 2nd generations of DAZ figures had 2 different meshes Victoria 1 & 2 has 1 while Mike 1 & 2 had a different one that was shared with the original Stephanie.
With the third generation, they used the same mesh for everything. This doesn’t mean that the SHAPE is the same but that they have the exact same vertices in the same position. This includes Victoria 3, Aiko 3, Stephanie Petite, David 3, Michael 3 & Freak 3. Compatibility was not 100% because the way the mesh was grouped for M3 & F3 was different from the others. All these figures also had very similar UV mapping. This was the first unimesh.
The 4th generation characters share another, newer unimesh. I think that the grouping for the 4th generation may be more consistent, but the UV mapping is less. (DS can switch UV maps it its necessary). Note that the Girl 4 is a unimesh figure while the original girl (Kim Goosens’) was not.
When clothing refers to unimesh fits, that is an expansion to make the clothing fit more characters.
http://www.daz3d.com/shop/v4-morphing-fantasy-dress/ is made to fit only Victoria4
http://www.daz3d.com/shop/morphing-fantasy-dress-unimesh-fits/ is an expansion that fits Aiko4, Stephanie4 and the Girl4 (1 or 2 also fit she freak but that is very rare)
Ditto for Mike/Hiro/Freak