A combination is always considered one obstacle, and the various elements within the combination are lettered "A", "B", "C", and so on. In cross-country, the rider need only retake the element they refused rather than the whole complex. So a refusal at element B does not require them to jump A again. However, they have the option of retaking the previous elements if they wish. For example, in a bounce type obstacle it may be physically impossible to approach B without first clearing A. Yet for some in and outs, you can go to B and not have to rejump A.