Hook: Wall Small (Green Square)The Damaged Wall Section decoration fits into a small green square wall hook.
Hook: Wall Medium (Blue Square)The Damaged Wall Section decoration fits into a blue medium square wall hook.
How to Get a Damaged Wall Section Decoration
Cartel Pack Decorations
The Damaged Wall Section decoration can be dropped from Underworld Alliance Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Underworld Alliance Pack, include a random selection of items from the Underworld Alliance collection of items. You aren't guaranteed to get a Damaged Wall Section decoration from a Underworld Alliance Pack, but it is one of the few ways to get a Damaged Wall Section deco apart from buying it on the GTN from another player.
If you're looking to get a Damaged Wall Section, the easiest way to get it is to head to the Galactic Trade Network (GTN) in SWTOR and buy it with credits from another player. If it's very rare and not available on the Cartel Market, you could try buying from the GTN and hope you get it.