If you include expenses in your contract by adding the project rule Include billable expenses in the contract amount, Core counts them against the contract amount even if you do not split the contract amount between services and expenses in Projects > General > Details. Splitting the contract amount here does not affect your invoices and time (or expense) evaluation. But it is used in the Contract Analysis widget in Projects > Performance and some reports. So it is still a best practice to split the contract amount, even though not required, if you are including expenses in the contract. Check Core Help for details.