The differentiation between two symmetry-allowed processes, for example the [2+4] vs. [4+6] cycloaddition of cyclopentadiene to tropone.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1150 (https://doi.org/10.1351/pac199466051077)