The purpose of identifying recommendation domains; must be for making recommendations from current data and for planning and interpreting future experiments. In a general sense every applied scientist must have a concept of a recommendation domain for his/her research. Some concept of the population to which results are relevant is integral to any research (even statistics!). The data on which the division of sites into groups for potential domains maybe some combination of (I) economic/sociological: based usually on surveys: (2) physical/meteorological/soils/vegetation: based on observation or on records from nearby available sources; (3) experimental results. There is also always potential for general qualitative judgment about site similarities. There are two stages to the identification of domains, which is a dynamic process rather than a permanent decision. We should separate the process of constructing groups from that of testing, or validating the group structure. The group construction may be attempted using any of the three forms of data. The validation process appears (to me) to be peculiar to experimental data because that data carries with it information about the precision of estimates calculated from the data. It would, of course, be possible to use the precision information inherent in experimental data to test groups (i.e. tentative domains) derived from other forms of data.41 pages