SAE doesn't have a definition for "full self driving", only levels of autonomy. "FSD" is term Tesla came up with to distinguish from their previous level 2 autopilot system which could only do highway driving, whereas "full" level 2 self driving can operate under all normal conditions, including city driving. FSD could theoretically cover levels 2, 3, 4, or 5. Highway-only could be levels 1, 2, 3, or 4. There's a lot of overlap.