Retail related classes run almost all the layers depending on functionality but CRT is the main layer for business logic.
CRT (.net dll): Business logic for example tax calculation, products fetch, info code, void transaction, etc. All major logic that communicate with channel database.
Retail Server (OData Services): CRT classes exposed as API for clients like POS and eCommerce.
Realtime service (X++) : Create Sales Order, Get real-time inventory, create customer, device activations etc.
POS Client (typescript) : Client side code for UI, messages, data binding, etc.
