Related Element fields let you connect two categories to each other. This field lets you manually select another element in another category to connect. This relationship works well if for example you want to connect a Manufacturer in your Contacts Category with a Furniture Spec in another category. Or if you want to connect Observation Notes to a Field Report in another category. Once you create a Related Element connection in one category, the other end of the connection will also be created in the other category. Related Elements are how manual Lookup fields are connected.