Aircraft with roller track cargo loading systems transport containers/pallets in the cargo compartments. When an aircraft doesn't consist of a certain system but just a flat floor (so for example a small bulk cargo aircraft), is it allowed (regulation wise), and if the dimensions are no problem, to place one or more (loaded) containers in the aircraft?
(I just saw you asked about Europe right as I posted this, if this is not applicable any more I will remove it)
If Im reading this correctly the question is asking "how is cargo secured if not loaded through the use of a Unit Load Device (ULD)"
Here in the US under the FAA there are some guide lines for tying down cargo if no ULD is present. You can read them here (SAFO 13008) and here (SAFO 13005). Much of the over arching information can be found in this Advisory Circular On Air Cargo Operations (AC 120-85).
...The manual must state the policy and provide procedures to determine the intended reaction load for each tie down to ensure sufficient special load restraint for flight loads...
You can tie it down as long as you conform to the above linked documents and ensure the straps and tie downs are sufficient to hold the cargo.