What is the load duration factor (CD)?

In section 2.3.2 of the NDS, it's stated that: Wood has the property of carrying substantially greater maximum loads for short durations than for long durations of loading. NDS Table 2.3.2 gives frequently used load duration factors. The 1997 NDS Commentary section 2.3.2 notes the following:

The characteristic behavior of wood structural members to carry a greater maximum load for short durations than for long load durations was reported as early as 1841. Load duration tests conducted on in-grade lumber in bending, tension, and compression and reported in 1986 and 1988 indicate the standard load duration curve is a generally adequate representation of the effect of load duration on strength. The long record of satisfactory performance with wood structures designed using load duration adjustment factors, and the results of load duration tests on full-size members, substantiate the general applicability of the standard strength-load duration relationship (NDS Appendix B). As more information is developed on the frequency of occurrence and the duration of maximum design loads specified in building regulations, use of different adjustment factors than those currently specified may be appropriate for individual load cases. Such changes would be associated with improved definition and recognition of the duration of the loads involved rather than a revision of the underlying relation between strength and time under load.