Holding is a special enchantment added by CoFH Core. It can be applied to certain items and blocks that can store items. The enchantment increases the amount of inventory slots in said items and blocks. The enchantment's maximum level is IV.
The Holding enchantment can be applied to Strongboxes and Satchels from Thermal Expansion. It increases the amount of slots in them by around 8-9 per enchantment level.
Note: Applying Holding V or higher to these items by using commands or similar may crash the game upon trying to open them.