Transactions.
Validators propose transactions for inclusion in the blockchain. Transactions have a rich structure. In fact, AleoBFT handles not only transactions, but also `solutions' and (possibly in the future) `ratifications', which together with transactions form `transmissions'.
However, these details are unimportant for our model. Our model sticks to the more common term `transaction', which can be thought as modeling also the other kinds of transmissions. We treat transactions as abstract entities.