Software defined optical transmission (SDOT) and sliceable programmable transceivers, exploiting multiple dimensions and photonic technologies, enable to support future networks with advanced and novel functionalities, meeting the capacity and reach targets, according to the segment requirements. Programmability and modularity are crucial for scalability and functional disaggregation, facilitating a migration towards flexible and disaggregated networking paradigms, where the infrastructure can be suitably sized, growing as needed. This work focus on SDOT enabling technologies, design guidelines and related trade-offs in terms of complexity and cost. Furthermore, SDOT programmability and modularity are discussed.