Classification trees are a visual way of identifying test objects and data, and pairwise testing is a technique of generating test cases that cover all possible pairs of values from different classifications. Classification trees support pairwise testing by:

Helping to identify inputs to be used by the pairwise test technique, as each class represents a possible value for a test parameter.

Showing the combinations needed to cover all-pairs, as each test case is a selection of one class from each classification.

Helping to identify particular combinations of interest, as the test engineer can use the tree structure to group or prioritize classes based on their relevance or risk. References = ISTQB Advanced Level Test Analyst Syllabus 2019, section 3.2.3; Smarter Test Design with Classification Trees and Pairwise Techniques, slides 6-12.