Relations connect Types inside a single App or across Apps.
Relations are very useful if you want to connect your data. Imagine you have Customers and you want to connect Customers to Features, Purchase Orders, Invoices, Requests, Issues, etc. Some of these Types are in different Apps, so it is very handy to use Relations to reduce data duplication. You can have a single list of Customers, connect it to required Types via Relations and select required Customer for every Feature, Invoice or Request.
Take a look at these Apps:
Every arrow is a relation — here we have plenty of them. For example, User Story is connected to Feature, so you can split a Feature to several User Stories:
Connect Types / Add new Relation
You can add new Relations from any Type. Check the diagram above, let's connect Features and Bugs. Every Feature will have many Bugs:
Now you can create new Bugs inside Features: