None.
The intent of the abort restart is to allow return to the innermost "command level." Implementors are encouraged to make sure that there is always a restart named abort around any user code so that user code can call abort at any time and expect something reasonable to happen; exactly what the reasonable thing is may vary somewhat. Typically, in an interactive listener, the invocation of abort returns to the Lisp reader phase of the Lisp read-eval-print loop, though in some batch or multi-processing situations there may be situations in which having it kill the running process is more appropriate.
Restarts, Interfaces to Restarts, invoke-restart, abort (function)