Invasion was the third stage in a planetary assault operation. Naval strategist Michael Unther argued that, while this stage which covered the ground battle for control of the planet was not the responsibility of the commander of the invasion fleet, it was a good idea to have some insight into this phase after all.
There were certain situations where the fleet could be called for to perform certain tasks such as taking out planetary shield generators, bombarding the planet into submission. Here, it was important to know which targets to destroy, like military targets, and which targets to avoid, like hospitals. The fleet could also be asked to clear landing zones; the density and nature of the bombardment depended on the units to be landed.
Invasions during a non-war situation were generally considered illegal during the time of the Old Republic. This was implied by the Trade Federation Viceroy Nute Gunray just prior to the Invasion of Naboo when receiving his orders from Darth Sidious to make preparations for invading the planet Naboo, where Gunray questioned the legality of such an action, with the Sith Lord making clear he'll ensure it was made legal.