HASONEVALUE Returns TRUE when the context for columnName has been filtered down to one distinct value only. Otherwise is FALSE.
Syntax
HASONEVALUE(columnName)
Parameters
ColumnName – The name of an existing column, using standard DAX syntax. It cannot be an expression.
Return Value
TRUE when the context for columnName has been filtered down to one distinct value only. Otherwise is FALSE.
Reference Data
https://docs.microsoft.com/en-us/dax/dax-function-reference