Non-uniformly distributed clouds in ultracool brown dwarf and exoplanet atmospheres cause the disk-integrated brightness to vary with rotation. With high-precision spectrophotometry, we can study the rotation-modulated brightness variation to understand the spatial heterogeneity of clouds and the vertical cloud structure, thereby unveiling the properties of three-dimensional clouds. Based on the color-magnitude diagram of more than thousands of brown dwarfs, we now know that cloud structural evolution plays a crucial role in causing the drastic color (J-H, J-Ks) change in the famous L/T spectral-type transition. In my talk, I will present the latest result of Hubble Space Telescope (HST) Large Treasury Program Cloud Atlas. I will show the high-precision time-series spectra of a rare mid-T planetary-mass object at the end of the L/T transition. With this object, we now have a small number of valuable low-gravity objects with detected rotation-modulated spectral modulation across the L/T transition. Based on the compiled samples with detected spectral variability, we report a tentative trend across the L/T transition on the color-magnitude diagram. This tentative trend may suggest a spectral-type dependence on the nature of rotational modulations.