What is the meaning of the 20-year period in Appendix C in the TRIM manual concerning the overall slope?

The idea of a slope leading to 20% loss in 20 years is to use a standard period in the interpretation of the trend estimate. Thus, even when one has data for e.g. a 5-year period, one still could interpret the results in terms of changes in 20 years. An overall slope based on a 5-year period that would result in an index of lower than 0.8 or higher than 1.20 after 20-years is regarded as a substantial decline or increase (in case the change is significant).