Returns returns the number of items in a particular path string. This function returns 1 for the path generated for an ID at the top/root of a hierarchy.
Learn more about PATHLENGTH in the following articles:
DAX does not directly support parent-child hierarchies. To obtain a browsable hierarchy in the data model, you have to naturalize a parent-child hierarchy. DAX provides specific functions to naturalize a parent-child hierarchy using calculated columns. The complete pattern also includes measures that improve the visualization of ragged hierarchies in Power Pivot. » Read more
Other related functions are:
Contributors: Alberto Ferrari, Marco Russo
MSDN documentation: https://msdn.microsoft.com/en-us/query-bi/dax/pathlength-function-dax