DAX function

RELATEDTABLE — DAX function reference

Get related rows (one-to-many).

Syntax

dax

RELATEDTABLE(<table>)

dax

COUNTROWS(RELATEDTABLE(Sales))

→ A table / count

dax

SUMX(RELATEDTABLE(Sales), Sales[Amount])

→ Total sales for the current row's product, in a calculated column on Product

dax

IF(COUNTROWS(RELATEDTABLE(Sales)) = 0, "Never sold", "Sold at least once")

→ Flags products with zero related transactions

RELATEDTABLE is the reverse of RELATED: it goes from the one side to the many side, returning matching related rows as a table — it's really CALCULATETABLE(Table) using the current row context as an implicit filter, so it also follows context-transition rules.
Like RELATED, it needs row context to know which 'current row' to filter by, so it belongs in calculated columns or as the table argument to an iterator, not directly in a plain measure.

From Relationships