Equity is what remains of the entity's assets once every liability has been deducted; an equity claim is any claim on the entity that is not a liability, and such claims can arise under a contract, a statute or something similar, and can come in several classes carrying different rights. Income is any rise in assets or fall in liabilities that lifts equity, except contributions by equity holders; an expense is any fall in assets or rise in liabilities that lowers equity, except distributions to equity holders. Both are thus derived from movements in assets and liabilities, and different transactions and events have different characteristics that users need to be able to distinguish.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.