There is no opacity, although one could use opacity for things like grates and fences for trims and floaters (in that case I’d use Masked). I’m not using decals that are floating above the other faces, rather using the faces that are already there (or cutting new ones into the mesh) to afford space for details. So it’s technically not the decal workflow but it allows for nearly all of the same freedoms as the decal workflow with some additional benefits.
The wireframed model below shows a circular floater toward the bottom. Its placement and size are determined by the geometry that’s there. To place it elsewhere on the model, I’d have to cut a square where I’d want the floater to be or use another square that exists on the model.
The reason why there are no seams is that the normal map, AO, etc. exist in a space separate from the grunge map. So UV0 is geometric hard-surface details, UV1 is grunge (simple unwrap, box-map, or you could even abandon UV1 for tr-planar projection).
The colors on the trim textures determine where the seams would be. So if the UV edge of a floater lands on a neutral zone (mid-gray for cavity map or white for AO) you never get a visible seam.