In the link below, or google Beer-Lambert low, you can find the answer. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Electronic_Spectroscopy/Electronic_Spectroscopy_Basics/The_Beer-Lambert_Law. There is a logarithmic relationship between the absorbance and the transmission/absorption. Transmission/absorption spectra presentation is for those interested in photonics (spectroscopy). Absorbance representation is for those interested on the concentration (chemists...).

The y-axis, which represents absorbance, is a unitless quantity because absorbance is defined as the logarithm of the ratio of transmitted to incident intensity. Dimensionally, the intensities cancel each other out, resulting in absorbance being unitless. Furthermore, normalizing the spectrum does not alter the unitlessness of the y-axis since the division factor does not affect the unit of absorbance. However, it is essential to report the data as normalized for the reader's clarity.

Normalization in experimental applications is context-dependent and lacks a one-size-fits-all approach. Various methods are employed tailored to the experiment’s needs. For instance, some may opt for normalization by dividing by the maximum value, while others prefer scaling between 0 and 1. Generally, both methods can produce comparable outcomes. However, in certain instances, they might result in a misinterpretation of the data. Therefore, it’s crucial to first determine the specific insights you aim to derive from normalization.