https://semiwiki.com/x-subscriber/clk-design-automation/4481-variation-alphabet-soup/

n response, foundries have broken out on-die variation as a separate component in their SPICE models. They created global corners for slow, typical and fast. These global corners, called SSG (slow global), TTG (typical global) and FFG (fast global), only include between wafer variance. On-die variance is separated out as a set of local parameters as part of the SPICE model that work with Monte-Carlo (MC) SPICE around the global corners. Analog designers routinely use these global corners and local parameters to validate cells. These same global corners and local variance parameters can be used to create derates or adjustment factors for static timing and physical optimization of digital designs (and the digital parts of mixed-signal designs).