For insulated cables the selection of conductor section is based on:
1) Thermal criterion. Parameters: design current, type of insulation, unipolar/tripolar, correction factor that depends on: ambient temperature, overhead/underground cable, type of lying (directly buried, buried under pipe, clearance between phases)
2) Voltage drop criterion. Each country has limits of voltage drop at MV and LV. Parameters: design current, conductor length, conductor resistance and inductance.
3) Shortcircuit criterion. The appearance of a fault must be thermally withstand by the insulator. Parameters: parameters of instantaneous trip of protection relay, equivalent impedance upstream.
For non-insulated cables the selection is based on:
1) Thermal criterion: maximum sag over ground, maximum annealing temperature of the aluminum
2) Voltage drop criterion: similar to insulated cable
3) Shortcircuit criterion. Similar to insulated cable
International standards: IEC 60287 for insulated cables. For non-insulated cables I do not know international standards, but national requirements.
Tripolar means that the three phases are together in the same cable (3-phase cable). On the other hand unipolar means that each phase is in an independent cable.