The definition of generations of physicochemical properties can be attributed to Hugh et al.  as recapitulated by e.g. Ferraz et al. :
The search for new and different ILs has led to the progressive development and application of three generations of ILs: 1) The focus of the first generation was mainly on their unique intrinsic physical and chemical properties, such as density, viscosity, conductivity, solubility,and high thermal and chemical stability. 2) The second generation of ILs offered the potential to tune some of these physical and chemical properties, allowing the formation of “task-specific ionic liquids” which can have application as lubricants, energetic materials (in the case of selective separation and extraction processes), and as more environmentally friendly(greener) reaction solvents, among others. 3) The third andmost recent generation of ILs involve active pharmaceutical ingredients (API), which are being used to produce ILs with biological activity.
The passage quoted in the OP stems from an article by Zakrewsky et al.  and refers to a study on a third generation IL. Third generation characteristics would in this case include those granting the IL antibacterial, nontoxic and transdermal delivery properties, that is allowing solubilization of drug compounds, minimizing damage to human cells, and disupting bacterial biofilms. In this case, "secondary" properties were not targeted, but these might include (as per the above quote) reduced environmental toxicity.
- Hough, W. L.; Smiglak, M.; Rodríguez, H.; Swatloski, R. P.; Spear, S. K.; Daly, D. T.; Pernak, J.; Grisel, J. E.; Carliss, R. D.; Soutullo, M. D.; et al. The Third Evolution of Ionic Liquids: Active Pharmaceutical Ingredients. New J. Chem. 2007, 31 (8), 1429–1436. https://doi.org/10.1039/B706677P.
- Ferraz, R.; Branco, L. C.; Prudêncio, C.; Noronha, J. P.; Petrovski, Ž. Ionic Liquids as Active Pharmaceutical Ingredients. Chem. Med. Chem. 2011, 6 (6), 975–985. https://doi.org/10.1002/cmdc.201100082.
- Zakrewsky, M.; Lovejoy, K. S.; Kern, T. L.; Miller, T. E.; Le, V.; Nagy, A.; Goumas, A. M.; Iyer, R. S.; Sesto, R. E. D.; Koppisch, A. T.; et al. Ionic Liquids as a Class of Materials for Transdermal Delivery and Pathogen Neutralization. PNAS 2014, 111 (37), 13313–13318. https://doi.org/10.1073/pnas.1403995111.