From TFA and AndyP's slides it seems to specifically refer to a variant of SSO where, for large strings, a fixed-sized prefix of the string is stored along-side the pointer, in the same location as that fixed-size prefix would be fore SSO strings. This means that strings lacking a common prefix can be quickly compared without pointer-chasing (or even knowing if they are small vs large).

All names given by people who were NOT of that nationality.