Interface indicating a operator implementation can be macro-fused back to Flowable in case the operator goes from Flowable to some other reactive type and then the sequence calls for toFlowable again:

Single<Integer> single = Flowable.range(1, 10).reduce((a, b) -> a + b);
Flowable<Integer> flowable = single.toFlowable();

The Single.toFlowable() will check for this interface and call the fuseToFlowable() to return a Flowable which could be the Flowable-specific implementation of reduce(BiFunction).

This causes a slight overhead in assembly time (1 instanceof check, 1 operator allocation and 1 dropped operator) but does not incur the conversion overhead at runtime.