The 9800 Series Wireless Controller uses CAPWAP tunnels to encapsulate and transport both control and data traffic between access points and the controller. This allows for centralized management and control while also enabling efficient data forwarding.
https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/config-guide/b_wl_16_10_cg/mobility.html

For a pre-deployment site survey, it's important to know the AP model to understand the capabilities and coverage pattern of the APs. Additionally, knowing the number of clients helps to design a network that can handle the expected client density.
https://www.cisco.com/c/en/us/support/docs/wireless/5500-series-wireless-controllers/116057-site-survey-guidel

By segmenting the VLAN into smaller subnets and assigning them by location, an engineer can use interface groups. This allows for efficient management of network resources and ensures that clients are connected to the closest subnet, reducing latency and improving performance.

To predict the number of APs needed to support approximately 300 wireless devices on a new floor with an
802.11ac network, the Demand Settings in the Cisco Prime Infrastructure Planning Tool must be adjusted.
This setting allows the planner to specify client density and throughput requirements, which are crucial for determining the correct number of APs.

Mobility domains and mobility groups are concepts used in wireless networking to manage client roaming and security contexts. When Wireless LAN Controllers (WLCs) are in the same mobility domain, they can communicate with each other, allowing clients to roam seamlessly between APs managed by different controllers. Mobility groups further refine this by constraining the distribution of a client's security context, which includes authentication and encryption keys, and by managing AP failover between controllers. This ensures that a client can maintain a secure and stable connection as they move throughout the network.
References: Designing Cisco Enterprise Wireless Networks (ENWLSD) 300-425 Official Cert Guide