In short
Slaby works out GST on every counter bill from the HSN code and rate you set on each item. For a business buyer, add their GSTIN: Slaby checks it, prints both GSTINs and the place of supply, and charges CGST + SGST for goods handed over at your counter, or IGST when you deliver to another state.
What is the difference between a B2C and a B2B GST bill?
A B2C bill goes to a customer without a GSTIN, like a walk-in buyer. A B2B tax invoice goes to a registered business and must show their GSTIN, so they can claim input tax credit.
On Slaby, every counter bill is a B2C bill until you add a GSTIN. Then the same bill becomes a B2B tax invoice with the buyer's legal name, address and GSTIN, and it is reported in the B2B sheet of your GSTR-1 file.
How do I add the buyer's GSTIN to a bill?
Open the B2B panel on the bill and type the 15-character GSTIN, for example 27AAPFU0939F1ZV. Slaby checks the format and the last check digit, so a typing mistake is caught before the bill is saved, and shows the state the GSTIN is registered in.
Add the legal name and address, and tick save to remember them for this customer next time.