3ds Max uses Smoothing Groups to create hard/soft edges between polygons, by splitting/combining vertex normals. When neighboring polygons do not share the same Smoothing Group, this creates a hard edge between them.
Maya uses Harden Edge and Soften Edge to do the same thing.
To reduce shading errors with tangent-space normal maps, it is best to split the UVs everywhere there are hard edges on the model.
However, hard edges are not needed for every UV edge. There can be more UV edges than there are hard edges, in fact more UV seams are often necessary to reduce texture warping.
Hard edges need to be separated in UV space, to allow the tangent basis to twist around appropriately. This creates less extreme gradients in the normal map (which reduces shading errors when using a non-synched renderer), and reduces errors introduced by LODs.
- You're making me hard. Making sense of hard edges, uvs, normal maps and vertex counts Polycount Forum thread
- turboTools for 3ds Max includes a tool to convert UV shell borders into hard edges.
- TexTools for 3ds Max includes a tool to convert hard edges into UV shell borders. The author also wrote a script to convert UV shell borders into hard edges, see [[3dsmax UVW seams to Smoothing Groups?]] Polycount Forum thread.
- EdgeSmooth for 3ds Max is a tool to convert selected edges into hard edges, acting much like Maya's workflow. However the resulting mesh creates shading errors in some game engines.
- PolyUnwrapper ($) for 3ds Max includes a tool to convert UV shell borders into hard edges.