Just Enough Items is a mod that adds a menu within your inventory. This menu will allow you to look at just about every item in RLCraft, how to craft each item and what you can craft with each item.
Item Catalog[]
In several inventory-based GUIs (such as when opening a container, your inventory, or managing item slots), you can find a catalog of items, as well as a search bar, to the right of the screen. Putting search terms into the search bar will narrow down the items and blocks that show in the catalog. Additionally, you can put a @ and the name of a mod (such as @AdvancedFishing, for example), and only the items/blocks from that mod will display.
Recipe Viewing[]
With JEI, you can view the recipes that create a specific item, as well as recipes you can create using said item. By hovering over an item and pressing R, you get the recipes of how to create that item. If no menu appears, it means that item cannot be created in a way that JEI registers. Also, you can hover over an item and press U to get the recipes of which that item is used in. (Another way of viewing recipes via the catalog is by left/right clicking items within that catalog.)
Usage[]
JEI is especially useful for modpacks such as RLCraft for viewing what materials are needed for advanced/complicated recipes, as well as finding useful weapons and armor for your current stage of playthrough. It can also highlight certain crafting methods normally not found in vanilla (such as brewing via Cauldrons), and the mod overall helps progression throughout the harshness of RLCraft.