The architecture Layer is the following:
User: represent the endpoints
Access: represent the storefront and Citrix Gateway
Control: represent the delivery controller, database, studio, license, director Resources: represent the VDAs Host: represent the hardware, fiscal server, storage, network, etc.
Page 33, 34 and 35from the hand book: https://docs.citrix.com/en-us/xenapp-and-xendesktop/7-15-ltsr/downloads/handbook-715-ltsr.pdf