Larry in TN
En-Route
An initial cruise altitude is filed on either type of flight plan.Anyway, back to the can two altitudes actually be ‘filed,’ it looks like it doesn’t happen on domestic flights, even those as long as all the way Puerto Rico.
The ICAO flight plan allows for changes in altitude to be filed within the routing section. Generally, particularly in the US, the controllers seem unaware of the altitude changes in the flight plan and they must be requested when you are ready for them.