Depending on your area, sawlogs generally are anything over 12" diameter inside bark on the small end of the log. Pulp goes down to 4". Sawlogs are generally graded, and the more clear wood, the better the value. Most of the value of the tree will be in first 16'.
For scaling purposes, trees are scaled with the number of 16' logs that are there. You can also use a ½ log. Pulp generally gets counted by the number of bolts. A bolt can be 4' or 5'.
When you get to metrics, things would probably change as to sawlog length or bolt length. It all comes out okay when you check your charts to get a total. Your scaling should yield the volume in sawlogs and the volume in pulp.