UNION Creates a union (join) table from a pair of tables.
Syntax
UNION(table_expression1, table_expression2 [,table_expression]…)
Parameters
Table_expression – Any DAX expression that returns a table.
Return Value
A table that contains all the rows from each of the two table expressions.
Reference Data
https://docs.microsoft.com/en-us/dax/dax-function-reference