If you have an H5P question set with three multiple choice questions inside it the Question Set will be the root content and each Multiple Choice question will be sub content. The id of the Question Set will be it's url and the id of the sub content will be it's url appended by its subContentId. The subContentId is a UUID. We could not use for instance the question number since that would result in a new idea for the question if the order of the questions were to change.
For convinience the sub content id is stored as property in xAPI statements from H5P in addition to as part of the ids.