Complex sentence: A group or segment can be mandatory (M) or conditional (C) and can be specified to repeat, for example C99 would indicate between 0 and 99 repetitions of a segment or group, whereas M99 would mean between 1 and 99 repetitions.
Split into simpler sentences: A group or segment can be mandatory (M) or conditional (C) and can be specified to repeat. For example, C99 indicates between 0 and 99 repetitions of a segment or group, while M99 signifies between 1 and 99 repetitions.