1. Physical Interface & Package Form Factor: The MMRD4 conforms to the standard SD card dimensions (approximately 32mm x 24mm), while the MURD4 utilizes the microSD card form factor (approximately 11mm x 15mm). This is not a superficial difference; the two are physically incompatible with their respective device slots. A standard SD card cannot be inserted into a microSD card slot. Although a microSD card can be used in an SD card slot via an adapter, introducing an adapter in industrial or embedded applications increases risks of connection unreliability, sensitivity to mechanical vibration, and additional space consumption, potentially violating the original design specifications. 2. Product Series & Application Scenario Tendency: The MMRD4 series (standard SD) is typically targeted at industrial equipment, embedded systems, or professional devices equipped with standard SD card slots. The MURD4 series (microSD) is more commonly used in space-constrained compact embedded modules, mobile devices, or as onboard storage chips. This divergence stems from their differing target PCB layouts and device mechanical structures at the design stage. Substituting one for the other may necessitate a redesign of the hardware interface and physical mounting solution. In summary, the selection depends on the device's interface specifications. Due to their fundamentally different physical form factors, they are not direct substitutes.