Each website as well as each store view must have a unique identifier.
These unique identifier are required to use the MAGE_RUN_TYPE and MAGE_RUN_CODE variables as below following steps
MAGE_RUN_TYPE can be either store or website
The Website is used to load a website in your storefront.
The Store is used to to load any store view in your storefront.
MAGE_RUN_CODE is the unique website or store view code that corresponds to MAGE_RUN_TYPE