True, I'm unclear on what precisely that means. C traditionally hasn't had subsetting, so "standard C" code is supposed to compile correctly on any conforming implementation. If your code is "standard C99", any conformant C99 compiler is supposed to compile it correctly; and otherwise, you can't assume that (for example, you may be consciously using a GCC extension, which is ok, or relying on implementation-defined behavior, which is usually bad).

But if you write C11 that uses VLAs, it seems like it's still standard C11, sort of: VLAs aren't merely a vendor-specific extension, like the GCC extensions are, but an optional C11 feature which hasn't been deprecated. Nonetheless, a conforming C11 compiler doesn't have to accept your code, so in that sense your code isn't really "standard C11". Will there be more fine-grained names for the different subsets of the standard features, so you can say that such code is compliant with "maxi-C11" but not with "core C11" or something?