DAX has two very similar functions, Related and RelatedTable which accomplish two very different actions.
We use these functions when we need to communicate to another table that we are not currently working on. These are quite often used in calculated columns and aggregations.
In this video, I show the difference between the two and hope to clear up any confusion you may have had in the past. Also, you will see some common use case scenarios of where these are used in Power BI Development. Bonus DAX functions thrown in will be SumX, AverageX, and RankX.