The spt file contains the procedural definition of the
tree. The definition parameters affect the features of
the tree. The actual 3D model of the tree is generated
by SpeedTreeRT at the game runtime.

For figuring out what tree features the parameters in
the spt file controls, we need a test tree which we
modify one parameter at a time. After each change
the tree is saved to a new file and the change is
documented. This creates a series of tree files.
For example,
  tree0.spt  original tree
  tree1.spt  Trunk/Radius Profile (was changed)
  tree2.spt  Branches/Level 2/Segments/Cross-section
  tree3.spt  Lighting/Leaf Lighting/Leaf lighting adjustment

The curve and parameter names can be found from the
SpeedTreeCAD application itself. But I have documented
them at
http://www.uta.fi/~majuko/speedtreecadnotes