The “strictly equal to” operator == returns TRUE when the two arguments have the same value or are both BLANK.
A comparison between BLANK and any other value returns FALSE.
This operator does not perform any implicit conversion between strings and numbers.
This comparison differs from = only when at least one of the two terms is BLANK, empty string, or 0.

The following are valid expressions using the “strictly equal to” operator.

Product[Color] == "Red"
Product[Color] == BLANK()   -- Returns TRUE if Product[Color] is BLANK, returns FALSE if it is "" or any other value
Product[Color] == ""        -- Returns TRUE if Product[Color] is an empty string, returns FALSE if it is BLANK or any other value
Sales[Quantity] == 1
Sales[Quantity] == BLANK()  -- Returns TRUE if Sales[Quantity] is BLANK, returns FALSE if it is 0 or any other value
Sales[Quantity] == 0        -- Returns TRUE if Sales[Quantity] is 0, returns FALSE if it is BLANK or any other value

The following are equivalent expressions using different comparison operators.

-- The following two expressions return the same result
AND ( <exp> = 0, NOT ISBLANK( <exp> ) )
<exp> == 0

-- The following two expressions return the same result
AND ( <exp> = "", NOT ISBLANK( <exp> ) )
<exp> == ""

There is no “strictly not equal to” operator (such as <> vs. ==), which can be obtained by using the NOT of the “strictly equal operator”:

<exp> == <value>               -- <exp> strictly equal to <value>
NOT ( <exp> == <value> )       -- <exp> strictly not equal to <value>

Despite the name of the operator, when it is used on text, the operator is usually case-insensitive, according to the collation used by the engine. The collation is always case-insensitive in Power BI and Azure Analysis Services. The collation can be modified during the installation of SQL Server Analysis Services and is case insensitive by default.

Compatibility: the “strictly equal to” operator == is available in Power BI, Azure Analysis Services, and Analysis Services 2019. It is not available in earlier versions of Analysis Services and in Power Pivot for Excel.

See Handling BLANK in DAX for more details about BLANK comparisons.