According to the Mist documentation1, WLANs are wireless networks that can be configured with different settings such as SSID, security, encryption, authentication, etc. WLANs can be created and managed at different levels of hierarchy in the Mist cloud.
The site group level allows you to create and apply WLANs to a group of sites that share common characteristics or requirements2. This helps you to simplify and standardize your WLAN configuration across multiple sites.
The site level allows you to create and manage WLANs for a specific site that may have unique needs or preferences1. This gives you more flexibility and granularity for your WLAN configuration at each site.