Create a Calculated Column from Different Tables using if formulae from Azure SQL Database

Be careful using calculated columns...the dramatically increase the size of your data model.  There's nothing inherently wrong with calculated columns, and it's a great way to create a column to use as an attribute (think slicer or something you want on the x-axis of a column chart).

I'm guessing you have a 1:* relationship between Code table and Transaction Posting table.

This code will create a calculated column in your Transaction Posting table with the value you seek:

Column =
IF (
    RELATED ( 'Accounts Nominal_Ledger_Code'[Nominal_Code] ) = 7001,
    /*This assumes that the [Nominal_Code] column is formatted as a number, use "7001" if it's formatted as text*/
    'Accounts Nominal_Ledger_Transaction_Posting'[Debit_Value]
        - 'Accounts Nominal_Ledger_Transaction_Posting'[Credit_Value],
    BLANK ()
)

This is the same logic but set up as a measure.  This will take all of the rows of the Transaction Posting table in the current filter context, and then apply the additional filter of Nominal Code = 7001.  It will perform the subraction that you described, and then add up each of those individual results.

[Measure] =
CALCULATE (
    SUMX (
        'Accounts Nominal_Ledger_Transaction_Posting',
        'Accounts Nominal_Ledger_Transaction_Posting'[Debit_Value]
            - 'Accounts Nominal_Ledger_Transaction_Posting'[Credit_Value]
    ),
    'Accounts Nominal_Ledger_Code'[Nominal_Code] = 7001
)

A final piece of advice.  Come up with user friendly names for your tables.  I recommend "LedgerCode" and "Transactions"  It will make your code much easier to follow.

[Measure] =
CALCULATE (
    SUMX (
        Transactions,
        Transactions[Debit_Value]
            - Transactions[Credit_Value]
    ),
    LedgerCode[Nominal_Code] = 7001
)

Column with OR =
IF (
    RELATED ( 'Accounts Nominal_Ledger_Code'[Nominal_Code] ) 
        IN {"Description 1", "Description 2", "Description 3", etc.},
    /*Add as many descriptions as you want inside quotes and separated by commas.  Note the CURLY BRACES to define this as a list.*/
    'Accounts Nominal_Ledger_Transaction_Posting'[Debit_Value]
        - 'Accounts Nominal_Ledger_Transaction_Posting'[Credit_Value],
    BLANK ()
)

[Measure] =
CALCULATE (
    SUMX (
        'Accounts Nominal_Ledger_Transaction_Posting',
        'Accounts Nominal_Ledger_Transaction_Posting'[Debit_Value]
            - 'Accounts Nominal_Ledger_Transaction_Posting'[Credit_Value]
    ),
    'Accounts Nominal_Ledger_Code'[Nominal_Code] IN  {"Code 1", "Code 2", "Code 3"...}
)

Same structure as the first measure I gave you.  I just used the

IN {"Code 1", Code 2", "Code 3"...} instead of = Code 1 approach.  IN works as a multiple OR statement.