A derivation scheme is just one of the ways to organize the storage or relationship of keys: each key in such a scheme will have its own place within this huge hierarchy, but they all can be derived from the common ancestor. The entropy (single signature wallet) or a bunch of entropies (multi-signature wallet) serve as an entry point from which to generate information needed for wallet creation and signing of transaction data. While you are absolutely free to determine how initial entropy and derived keys should relate to each other, it is advisable to follow standards described in BIPs and similar specifications because this helps wallet software to remain interoperable and interchangeable.