Updated to add agreement id and customer id

Author: clairekinde11

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

@@ -21,6 +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
+
+- **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.
+
 ## Key concepts
 
 - **Plan groups** - Plan groups organize multiple plans under a common category, helping segment offerings by use case, customer size, or market. A group is scoped to either organizations (B2B) or individual customers (B2C). You can have multiple plan groups, but each plan must belong to a group.