Lightmap generation process

The lightmapA pre-rendered texture that contains the effects of light sources on static objects in the scene. Lightmaps are overlaid on top of scene geometry to create the effect of lighting. More info
See in Glossary generation process is as follows:

• UV Mapping: By default, Unity uses the meshThe main graphics primitive of Unity. Meshes make up a large part of your 3D worlds. Unity supports triangulated or Quadrangulated polygon meshes. Nurbs, Nurms, Subdiv surfaces must be converted to polygons. More info
  See in Glossary’s imported UVs. You can also make Unity generate a per-mesh UV layout with UV charts for each mesh. A margin separates charts within the same layout.

• Packing: Unity packs multiple UV layouts into a shared UV atlas. Padding between different UV layouts in the atlas optimizes texture space and prevents bleeding.

• Texture generation: Unity converts the UV atlas into the final lightmap texture that contains the pre-calculated lighting data.

Unity uses real-time lightmap UVs for the Realtime Global IlluminationA group of techniques that model both direct and indirect lighting to provide realistic lighting results.
See in Glossary system and baked lightmap UVs for the Baked Global Illumination system. There are two reasons for this:

• There’s no direct correspondence in the grouping of instances between real-time and baked lightmaps; instances that are in the same real-time lightmap might be in two different baked lightmaps, and vice versa.
• Meshes that appear at different scales share lightmap UVs in baked lightmaps, but don’t share UVs in real-time lightmaps.