Today, optical transport and data center networks extensively utilize photonic integrated systems due to their large bandwidth and a high degree of reconfigurability. In addition to these properties, photonic integrated-based systems can deliver an overall low fabrication cost, a small footprint, and low power consumption. In this perspective, we present a modular photonic integrated multi-band wavelength selective switch (WSS) capable of managing a wide spectrum, covering the three S+C+L bands, and potentially scalable to larger numbers of output fibers and routed channels. We propose a complete description of the device starting from the physical level, commenting on the device's internal structure and design-related issues. Then, we move to the transmission level, providing a complete abstraction of the proposed WSS in the context of software-defined optical networks by providing a deterministic model to evaluate the routing controls, thermal spectral tunability, and the quality of transmission degradation. Finally, a transmission scenario operating on 400ZR standards and a network case study are also demonstrated to evaluate the performance of the proposed WSS in a single or multistage cascade setup.