An agent VM is a special type of VM that is used for running services or applications that are not affected by high availability (HA) events or host maintenance. An agent VM is never migrated to any other host in the cluster. If an HA event occurs or the host is put in maintenance mode, agent VMs are powered off and are powered on on the same host once that host returns to a normal state. This behavior is different from regular VMs, which are migrated to another host in the cluster if possible, or restarted on the same host if not possible1.
Therefore, during an AHV upgrade, an administrator may find that a critical VM was powered off rather than migrated to another host if the VM was marked as an agent VM. This can be done either from the Prism web console by selecting the Use this VM as an agent VM option in the Update VM dialog box, or from the aCLI by using the vm.create or vm.update commands with the agent_vm=true parameter2. To avoid this situation, the administrator should ensure that any critical VMs are not marked as agent VMs before starting the AHV upgrade.

Prism Central is a multi-cluster manager that provides a single, centralized management interface for Nutanix environments1. Prism Central can be deployed in different configuration scenarios, depending on the network and security requirements of the environment. Some of the valid scenarios are:
Environments use Network Address Translation (NAT): NAT is a method of mapping one IP address space to another by modifying network address information in IP datagram packet headers while they are in transit across a traffic routing device2. NAT can be used to enable communication between Prism Central and Prism Elements that are in different networks or subnets3. For example, Prism Central can be deployed in a public cloud and use NAT to access Prism Elements that are in a private data center3.
Prism Elements and Prism Central are in different subnets: A subnet is a logical subdivision of an IP network that allows multiple networks to share a single physical network4. Prism Elements and Prism Central can be in different subnets as long as they can communicate with each other through routing or NAT3. For example, Prism Central can be deployed in a management subnet and access Prism Elements that are in different application subnets3.
Environments do not have Internet access: Internet access is not required for the deployment of Prism Central, as long as the environment meets the prerequisites and considerations for installing or upgrading Prism Central. For example, Prism Central can be deployed in a dark site, which is an environment that does not have Internet access or has restricted Internet access. In this case, the administrator needs to manually enable microservices infrastructure and download the required software packages from another source.

Curator is the process that runs on every node in a Nutanix cluster and is responsible for data management tasks such as deduplication, compression, erasure coding, and replication factor compliance. Curator also handles disk failure recovery by scanning Cassandra to find all data previously hosted on the failed disk, and all disks in that node. Curator then rebuilds the data on other nodes in the cluster using the distributed storage fabric1.

https://portal.nutanix.com/page/documents/details/?targetId=Web-Console-Guide-Prism-v51:wc-block-services-c.html