They all have a customer, a list of items and a total. That is where the similarity ends. Here is what each one actually is, in the order you normally need them.
Quotation — before anything is agreed
A price you are offering. It commits nobody, creates no receivable, and belongs in no return. Its only job is to let the customer say yes or no. If your software turns quotations into invoices automatically, that is a convenience, not the same document.
Proforma — after they say yes, before you supply
The most misunderstood of the five. It looks like an invoice and is not one: it is a statement of what the invoice will say, usually sent so the customer can arrange payment or approval first. It is not a tax document and should never be treated as one by either side.
Tax invoice — the actual document
This is the one that matters. It records a supply that happened, carries the tax, and is what your books and your customer's input credit both rest on. Everything before it is preparation; this is the event.
Credit note — you are reducing what they owe
A return, a shortfall, an agreed discount after the fact, a correction downward. It is not a deletion of the invoice and should never be used as one. The original invoice stays; the credit note records the adjustment against it, which is exactly what makes the history readable later.
Debit note — you are increasing it
The mirror image: something was undercharged, or extra was supplied. Less common, and for that reason more often done wrong — usually by raising a second invoice instead, which leaves two documents where the relationship between them is invisible.
The mistake that costs the most time
Editing or deleting an issued invoice instead of raising a credit note. It feels tidier and it destroys the audit trail — your copy and the customer's copy now disagree, and nobody can tell what changed or when. A credit note is longer to explain and shorter to live with.
How this looks in BizGST Pro
All five are the same screen, chosen with a tab at the top. Same customer, same item lines, same arithmetic — the document type changes what it is called, what prints on it, and how it is treated afterwards. One place, not five.
What this article does not cover
When each document is required, what must appear on it, and how any of it is treated for tax. Those are set by rules that change, and the sources are the GST portal and your accountant. What is above is the difference in meaning, which does not change.