tidb-cloud: update the RU definition

tidb-cloud/tidb-cloud-glossary.md

@@ -145,7 +145,12 @@ A Request Capacity Unit (RCU) is a unit of measure used to represent the provisi
 
 ### Request Unit (RU)
 
-A Request Unit (RU) is a unit of measure used to represent the amount of resources consumed by a single request to the database. The amount of RUs consumed by a request depends on various factors, such as the operation type or the amount of data being retrieved or modified. For more information, see [{{{ .starter }}} Pricing Details](https://www.pingcap.com/tidb-cloud-starter-pricing-details/).
+For {{{ .starter }}} and Essential, a Request Unit (RU) is a unit of measure used to represent the amount of resources consumed by a single request to the database. The amount of RUs consumed by a request depends on various factors, such as the operation type or the amount of data being retrieved or modified. However, the billing models for {{{ .starter }}} and Essential are different:
+
+- {{{ .starter }}} is billed based on the total number of RUs consumed. For more information, see [{{{ .starter }}} Pricing Details](https://www.pingcap.com/tidb-cloud-starter-pricing-details/).
+- {{{ .essential }}} is billed based on the number of provisioned [Request Capacity Units (RCUs)](#request-capacity-unit-rcu). One RCU provides a fixed amount of compute resources that can process a certain number of RUs-per-second. For more information, see [{{{ .essential }}} Pricing Details](https://www.pingcap.com/tidb-cloud-essential-pricing-details/).
+
+For TiDB Cloud Dedicated and TiDB Self-Managed, a Request Unit (RU) is a resource abstraction unit that represents system resource consumption, which currently includes CPU, IOPS, and IO bandwidth metrics. It is used by the resource control feature to limit, isolate, and manage resources consumed by database requests, **not for billing purposes**. For more information, see [Use Resource Control to Achieve Resource Group Limitation and Flow Control](/tidb-resource-control-ru-groups.md).
 
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