SELECTCOLUMNS Adds calculated columns to the given table or table expression.
Syntax
SELECTCOLUMNS(table, name, scalar_expression [, name, scalar_expression]…)
Parameters
table – Any DAX expression that returns a table.
name – The name given to the column, enclosed in double quotes.
expression – Any expression that returns a scalar value like a column reference, integer, or string value.
Return Value
A table with the same number of rows as the table specified as the first argument. The returned table has one column for each pair of name, scalar_expression arguments, and each expression is evaluated in the context of a row from the specified table argument.
Reference Data
https://docs.microsoft.com/en-us/dax/dax-function-reference