Deriving a computed column based on several nullable columns
You could use the CONCAT function in SQL Server 2012 or later, which automatically ignores NULL, and implicitly converts the inputs to string types if necessary.

Example:

CREATE TABLE #Demo
(
    [Tag Type] char(1) NOT NULL,
    [Parent Tag Type] char(1) NULL,
    [Tag Area No] integer NOT NULL,
    [Tag seq No] integer NOT NULL,
    [Tag Suffix] char(1) NULL,

    [Tag] AS 
        CONCAT([Tag Type], [Parent Tag Type], [Tag Area No], [Tag seq No], [Tag Suffix])
);

INSERT #Demo
    ([Tag Type], [Parent Tag Type], [Tag Area No], [Tag seq No], [Tag Suffix])
VALUES
    ('T', NULL, 1234, 1, NULL),
    ('T', 'P', 5678, 1, 'S');

SELECT * FROM #Demo AS D;


Output:



Tag Type | Parent Tag Type | Tag Area No | Tag seq No | Tag Suffix | Tag     
:------- | :-------------- | ----------: | ---------: | :--------- | :-------
T        | null            |        1234 |          1 | null       | T12341  
T        | P               |        5678 |          1 | S          | TP56781S



... and just in case the example was simplified and a separator is to go between the elements, check out CONCAT_WS() introduced in SQL Server 2017.