All conditions serious enough to require interactive intervention if not handled should inherit from the type serious-condition. This condition type is provided primarily so that it may be included as a superclass of other condition types; it is not intended to be signaled directly.
Signaling a serious condition does not itself force entry into the debugger. However, except in the unusual situation where the programmer can assure that no harm will come from failing to handle a serious condition, such a condition is usually signaled with error rather than signal in order to assure that the program does not continue without handling the condition. (And conversely, it is conventional to use signal rather than error to signal conditions which are not serious conditions, since normally the failure to handle a non-serious condition is not reason enough for the debugger to be entered.)