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Bousquet, L., Lachérade, S., Jacquemoud, S. and Moya, I., 2005. Leaf BRDF measurements and model for specular and diffuse components differentiation. Remote Sensing of Environment, 98(2-3): 201-211. Specular reflectance originates from leaf surface. On the other hand, diffuse reflectance originates from leaf intrastructure. To focus on the target traits, the two reflectance needs to be separated.

Li, W., Sun, Z., Lu, S. and Omasa, K., 2019. Estimation of the leaf chlorophyll content using multiangular spectral reflectance factor. Plant Cell Environ, 42(11): 3152-3165. Leaf reflectance can be affected by leaf BRF as well as leaf pigment absorption. In remote sensing, index which is not sensitive to surface angel has to be developed.

Hovi, Aarne, et al. "Evaluation of accuracy and practical applicability of methods for measuring leaf reflectance and transmittance spectra."  Remote Sensing  10.1 (2017): 25. Leaf clip can sense only certain angular light. The reflectance significantly differs from integrating spheres which can sense hemisperical radiant flux, as leaf surface is not assumed as a lambertian surface.

Schaepman-Strub, Gabriela, et al. Reflectance quantities in optical remote sensing—Definitions and case studies. Remote sensing of environment 103.1 (2006) 27-42. If a surface was not an ideal specular or diffuse surface, one could observe diffuse light as well as specular light reflected off the surface. Reflectance is affected by where the incident light comes from and where the light is observed, which is represented by an angular distribution function. So different reflectance concepts are possible, so it is needed to use the term reflectance practically.