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>