Today, each Nimiq transaction states the block from which it is valid, for a limited window. The alternatives are the nonce model, where every account keeps a counter (as in Ethereum), and the UTXO model, where your balance is a set of individual “coins” and each payment spends specific ones (as in Bitcoin).
The current approach proved difficult during the bridge work, and an account-based design makes Lightning-style payment channels hard. The design for how transaction states are done needs to be finalized to enable future work on the Blockchain