Why is GST required for generating e-invoices in BUSY? How do I configure that?

Want To Know Why GST required for generating e-invoices in BUSY & want to configure that:
 1.As Per Government Rule You Can Generate Einvoice only for B2B Transactions. So Its Mandatory To enter GST Number in Account Master.
 2.In Account Master Add The GST Number.
 3.Go to the Administration Select Configuration Then click on Features & options.
 4.Then Click on GSP Configuration & Select Automatically.
 5.Then Generate GSP User Id or Password. For it Open your Eway Bill Portal on Any Browser & Login Your Id.
 6.Then in Menu Option Click on Registration Then Click on For GSP.
 7.Then Click on Send OTP. Then Enter the OTP You Received on your Registered Number or Email Id.
 8.Then Select User As Webtel Electrosoft Private Limited.
 9.Then Enter Any 3 Character.
 10.Then Generate Password & Save the ID.
 11.Then Generate another GSP Id of Pinnacle Finserv Advisors Private Limited.
 12.Then Follow Step 9 & 10. 
 13.Then Copy Your Webtel User Id & Enter it on GSP Configuration in BUSY on Webtel User Id Then enter the Password You Generated on Portal & Click on Validate.
 14.Then Copy Your Pinnacle User Id & Enter it on GSP Configuration in BUSY on Pinnacle User Id Then enter the Password You Generated on Portal & Click on Validate & Save the Page.
 15.Then Go to the Administration Select Configuration Then click on Voucher Series Configuration.