To define a multicore in VeSys Design Classic you need to use the 'modify wire' menu option and define the multicore id under the 'more properties' tab. All wires within the same multicore must be assigned the same multicore Id. All wires within a multicore have to be of type wire, screen or fly lead.
The Assign multicore function allows you to assign a multicore indicator symbol to the multicore.
If you have a complex multicore you will have to set up a library part number that reflects the various cables that will make up the multicore. Assign the same multicore ID to each wire making up the complex multicore then go to Design/Modify multicore, select the multicore id and browse for the relevant library part number. You will then be able to select each wire id in turn and link it to the relevant wire defined in the library part.