As you begin your hosting reselling business, you should determine how you will offer your hosting resources and services to customers. Plesk allows you to organize your business using service plans and subscriptions.

A service plan is a combination of hosting resources that you sell to your customers. For example, a plan might include the web hosting service, 15 GB of disk space on the server, and 50 GB of data transfers per month.

Add-on plans allow you to offer additional resources and services to subscribers, such as an additional GB of disk space.

Customers who want to host a website on a Plesk-managed server can subscribe to a plan that offers the resources they need. In other words, a subscription is a set of resources defined by a plan and made available to a particular customer.

The Properties of Hosting (Add-on) Plans and Subscriptions section provides a complete list of Plesk server resources and services that you can include in your hosting plans.

Allocation of Resources and Services

Service plans define potential services; however, Plesk does not verify if a service or resource that a service plan should provide is actually available, either in the system overall or in your reseller subscription. For example, when creating a plan, you may choose to allow subscribers to connect to the server shell over SSH even if your subscription does not provide the Management of access to the server over SSH privilege. In this case, Plesk will allow you to do so and will not display any error or warning messages.

Subscriptions provide actual resources and services that are allocated upon the creation of each subscription.

Relations Between Subscriptions and Service/Add-on Plans

Normally, a subscription is associated with a service plan, as reflected in a list of subscriptions where each subscription name includes the service plan name in parentheses at the end. The resources and services provided by a subscription can be extended by associating the subscription with add-on plans. A subscription can be associated with multiple add-ons, but each add-on can only be added to the subscription once.

It is also possible to have subscriptions that are not associated with any service plans or add-on plans. These subscriptions are called custom subscriptions and are identified as such in the list of subscriptions by having their names extended with ‘(Custom)’. Custom subscriptions may be useful if you want to offer services with terms that differ from your typical business model.

At any time, you can change the association between a subscription and plans in the following ways:

• Associate the subscription with another service plan.
• Add and remove add-on plans.
• Remove the subscription association with the service plan and add-ons.

Syncing Service Plans and Subscriptions

When you modify the properties of a service plan, Plesk automatically applies the changes to all subscriptions associated with that plan. If you want to prevent a specific subscription from being automatically updated, you can lock the subscription.

Refer below link 

https://www.srilankahosting.lk/knowledgebase/syncing-subscriptions-and-plans/