The Level 3 event lists are compiled mostly by visual inspection. Jian et al. (2018, 2019) describe the methodology including the identification criteria. 

L. K. Jian, C. T. Russell, J. G. Luhmann, A. B. Galvin, STEREO Observations of Interplanetary Coronal Mass Ejections in 2007-2016, The Astrophys. J., 885, 114, doi: 10.3847/1538-4357/aab189, 2018.
L. K. Jian, J. G. Luhmann, C. T. Russell, A. B. Galvin, Solar Terrestrial Relations Observatory (STEREO) Observations of Stream Interaction Regions in 2007˗2016: Relationship with Heliospheric Current Sheets, Solar Cycle Variations, and Dual Observations, Solar Phys., 294, 31, doi: 10.1007/s11207-019-1416-8, 2019.