Subitems allow for Items to be broken up into smaller pieces, each contributing to the Parent Item. You can add them in the ‘Relations’ section within the Item modal. When you click to add a Subitem you’ll have the choice to choose an existing Item (search bar will help you find the one you want) or to create a new one (an empty card will appear just like when you’re creating a new Item from the Backlog). You can add as many Subitems as you need to.
If you’ve already created an Item that you want to turn into a Subitem, you can open it and in the ’Relations’ section click on the red ‘Add parent’ button. You’ll be asked to search for an Item where you want to add a Subitem.