[Celestial Guardian]

Basically what Romulas is trying to say is that the drones are a good way to allocate hits to places other than taking them directly on the suits. By doing this the drone will die, but you still have a valid unit on the board.

If the suit dies, then the drone also goes becaus it is used as part of that suits equipment, so no other suit may use it.

The hits can be allocated to the drones until they fail a save, then you would allocate the remaining hits to the suit. Example: Suit a has two shield drones. The unit is being shot at and recieve 6 hit which require saves. The profile for any drone matches the strength an toughness for the suit it is paired with (ie made of the same materials). The tau player may take shot and allocate them to the two sheild drones until they are both destroyed. In the rolls 3 hits are saved and three fail. Therefore both shield drones are now destroyed, but the suit still has a wound it can take.

Most people I know go crazy on the shielding. I have seen sheild drones with a suit that has a sheild generator. It's legal becaus the equipment my serve the same purpose, but the model has not taken the same equipment twice.

Even when using gunnery drones, the numbers to allocate hit to helps the suit survive, in gameplay this is their primary function.

Codex Tau Empire page 31. The boxed section explains the uses of every drone available in the codex.