Intel determines a processor's TDP by running a few dozen workloads on the product and measuring thermal dissipation/power consumption. These workloads include individual applications, multitasking workloads (CPU + GPU for example) and synthetic measures that are more closely related to power viruses (e.g. specifically try to switch as many transistors in parallel as possible). The processor's thermal behavior in all of these workloads ends up determining its TDP at a given clock speed.
Scenario Design Power (SDP), on the other hand, is specific to Intel's Y-series SKUs. Here Intel takes a portion of a benchmark that stresses both the CPU and GPU (Intel wouldn't specify which one, my guess would be something 3DMark Vantage-like) and measures average power over a thermally significant period of time (like TDP, you're allowed to violate SDP so long as the average is within spec). Intel then compares its SDP rating to other, typical touch based workloads (think web browsing, email, gaming, video playback, multitasking, etc...) and makes sure that average power in those workloads is still below SDP. That's how a processor's SDP rating is born.
