adds additional sections

Author: ZPain8464

content/docs/concepts/clusters.mdx

@@ -14,9 +14,7 @@ import TabItem from '@theme/TabItem';
 
 This document describes Clusters in Pomerium Zero.
 
-TODO: include a video that walks through Clusters
-
-## Clusters overview
+## Overview
 
 A **Cluster** represents a single Pomerium deployment managed by Pomerium Zero, our hosted control plane. You can think of a Pomerium cluster as a completely distinct deployment of Pomerium that allows you to effectively shard your deployments for performance, governance, security, or management purposes.
 
@@ -28,13 +26,22 @@ When you deploy a cluster, it gets its own:
 - **Routes and Policies**
 - **Wildcard certificates**
 
-### Cluster use cases (Why clusters?)
+### Cluster use cases: Why clusters?
+
+#### All-in-one mode deployment model
+
+Pomerium consists of 4 services:
 
-TODO: Fill out reasons why a clusters model makes sense
+- Proxy service
+- Authenticate service
+- Authorize service
+- Databroker service
 
-TODO: It would be ideal to get an architecture diagram here.
+A cluster deploys these services in all-in-one mode, meaning the configuration for each service is managed from a single cluster instance. All inter-component communications are internally connected, with the exception of the Databroker service, which is an external dependency.
 
+#### Multiple cluster instances
 
+You can deploy multiple clusters from a single cluster identity (see [**Cluster identity**](#cluster-identity) below for more information). The ability to deploy multiple cluster instances means you can manage and run completely separate configurations for workloads that may otherwise be incompatible in a single cluster instance.
 
 ### Cluster identity
 
@@ -126,12 +133,20 @@ curl https://console.pomerium.app/install.bash | env POMERIUM_ZERO_TOKEN=<cluste
 
 ### Cluster domain
 
-Pomerium Zero generates and assigns a unique **Cluster Domain** to your cluster. A cluster domain contains a randomly generated subdomain and the `pomerium.app` domain.
+Pomerium Zero generates and assigns a unique starter **Cluster Domain** to your cluster. A cluster domain is a randomly generated fully qualified domain name that follows this format:
 
-It follows this format: `<CLUSTER-SUBDOMAIN-XXXX>.pomerium.app`
+ `<CLUSTER-SUBDOMAIN-XXXX>.pomerium.app`
 
 For example: `voracious-ape-1578.pomerium.app`
 
+The starter cluster domain comes with its own DNS records and TLS certificates so you can quickly test Pomerium Zero before adding your own custom domain.
+
+:::info
+
+See the **Custom Domains** capabilities page for more information on how to add custom domains to a cluster.
+
+:::
+
 #### Cluster name
 
 The **Cluster Name** is a customizable identifier for your cluster. It defaults to the randomly generated subdomain of your cluster domain, but you can change it at any time. Changing the cluster name does not affect your cluster domain.