docs: correct payment and fee semantics

CONTEXT.md

@@ -20,6 +20,22 @@ One payment journey identified by the source session key. A session belongs to o
 
 One try within a payment session, distinguished by its try sequence. This is the grain of a source CSV row and must not be confused with a unique payment session.
 
+## Bank-Paid Session
+
+A payment session whose source status is `Paid` or `Verified`. In both states the buyer completed payment at the bank and funds were deducted. This is the numerator population for payment success and paid-volume metrics.
+
+## Verified Session
+
+A bank-paid session whose merchant verification step is complete (`Verified`). It is finalized, and is a subset of bank-paid sessions.
+
+## Unverified Paid Session
+
+A bank-paid session whose bank step succeeded but whose merchant verification step has not completed (`Paid`). It is successful at the bank, not a failure, and must remain visible as an actionable verification gap.
+
+## Merchant Fee
+
+The Rial fee associated with one payment session. The challenge source calls it `adjusted_fee` and repeats the same value on each attempt row; the normalized model stores it once on the session as `fee_amount_rial`.
+
 ## Opaque Payer Token
 
 The nullable `payer_card_key` source value. It is an unverified, merchant-scoped token used only for internal approximate repeat-payer analytics. It is not a card number or verified person identifier.