Some meshes (eg. tree trunks) use a lot of tiling textures that are repeated many times vertically up the mesh. Baking this tiling would usually exceed the maximum atlas size and atlas quality would be poor.
It would normally be impossible to pack these textures together effectively in an atlas. However it is still possible to pack them if the textures are packed side-by-side and stretch to the top and bottom of the atlas.
Meshes that use this atlas can tile in the vertical direction but not the horizontal direction.
This is what the Texture Packer: Mesh Baker Texture Packer Vertical does. Any meshes that tile vertically are packed side by side and touch the top and bottom of the atlas.
Texture Packer: Mesh BakerTexture Packer Horizontal Does the same but in the horizontal direction.
Any extra textures that do not tile vertically or horizontally are packed the regular way.