I.I. is short for ‘Ionic Inducing’ which means a melee weapon is capable of both piercing and often damaging shields.
Shields work on a combination of magnetic principles and maintain a strong balance of various ion and particles fields. They do not interact in normal day-to-day life and only when attacked or hit by hostile weaponry. Due to a speed limitation of sorts on shields, it IS possible a completely normal weapon such as a bat could ‘get through a shield’ (so much as not activating it) but this is rare as many shields have basic A.I. components of determining both minimum speed and hostile actions.
The long and short way through all of this is weapons that radiate a specific gamut of ions that over-load shields and allow the weapon to strike through the shield. Depending on the technology these can be anything from simply allowing hits THROUGH the shield (hoping to kill the enemy) to outright damaging/overloading the shield.
Shields have revolutionized combat and made nearly all conventional forms of fighting nearly extinct if the target has a shield. In general a fight might still open up with more normal weaponry on the chance the target does not have a shield. Many skip this and go straight to melee range.
This tremendously changed fighting, going back to a hybrid war of sorts where a majority of deaths came from melee range. Two groups shooting at each other would likely only take a few people out and that is with heavily concentrated fire on specific targets. The group that had shields and melee I.I. weapons would be upon the shooting group and quickly dispatch them all.
With an I.I. weapon and a shield themselves, the only way, short of heavy bombardment, they can be defeated is through a superior melee opponent.
Usage in space?
As discussed in the shield page, shields are strong on an individual level, when going against individual weapons. Starships have tremendous power and even with larger shield generators shield can be ‘shot through’ easier. As a result ‘melee range’ is not realistic and not necessary as long as there is sufficient munitions on the ship.
Due to needing a transmitter of sorts, it is simply not practical to have weapon rounds that have I.I. components on them. It IS possible, just largely impractical. I.I. technology is expensive, and somewhat large. It would be the equivalent of shooting a armor-piercing tank round at a single human. Some people carry limited specialized shells that cause distortions in shields that can be shot through. These are expensive and only used in special circumstances.