Added stripe customer number

Author: clairekinde11

src/content/docs/billing/about-billing/billing-concepts-terms.mdx

@@ -21,10 +21,11 @@ deprecated: false
 
 'Billing' refers to the broad function of creating plans, setting pricing, collecting payments, etc. Here are some of the common concepts and terms you will come across.
 
-## In-app identifiers
+## Billing identifiers
 
 - **Customer ID** - A `customer_id` uniquely identifies a user or org who has signed up to a plan. This is different to a `user_id` or `org_id` that identifies the user or org as an entity. 
 - **Customer Agreement ID** - A `customer_agreement_id` identifies the contract or agreement created in Stripe when a plan is signed up to. The agreement ID is then associated with the `customer_id` in Kinde.
+- **Stripe customer number** - A customer number is automatcially generated in Stripe when a customer signs up to a plan. You don't need this number to interact or do anything in Kinde. The Stripe customer number looks like this - `cus_SobqVb484c2xxx`.
 
 ## Key concepts