The most valuable part of a standing tree is the butt log, or the first 16-ft log above the stump on the main bole. Sawlogs and veneer logs are cut to various lengths depending upon the location of defects on the external face of the logs, diameter inside bark (DIB), product class, and market segment demand.
Logs are frequently bucked to lengths of 8'6", 9', 10', 12', 14' -- certain markets may want/demand logger log lengths, such as the timber mat construction market. Granted, shorter logs can be cut and sold -- again, depends on the buyer and the market. Oh, and don't forget DIB, or diameter inside bark, when bucking your logs for grade!
Uppers, or top logs, and limb logs are bucked via the exact same fashion as butt logs and lowers. Uppers tend to have a little more sweep, smaller diameters, more external face defects, and shorter log lengths compared to butt logs.
Please refer to these Extension Forestry publications for additional information:
Univ. of Tennessee, “A Hardwood Log Grading Handbook”
Purdue Univ., “Buck your Logs to NHLA Rules”