This is used when the Tax amount is included in the Sales value and the same is illustrated with an example.
You can either alter the existing Sales Voucher Type or create a new voucher type based on their requirements.
Go to Gateway > Accounts Info > Voucher Types > Alter > Sales
Give a name to the class under Name of Class say Sales Class B. You can create more than one class. Specify the table of class as below:

Now pass Sales Invoice and check,
Go to Gateway of Tally >Accounting Vouchers > F8 Salesselect Sales Class B

The total value of the sale is Rs. 1500/=. Select the entry in alteration mode, move the cursor till the amount field and check.

The total value is allocated to Sales and Tax Ledgers, as defined in the voucher class.