The three common CPQ design mistakes to avoid while implementing CPQ for your customer are:
A: Using price book entries to handle attribute based variations instead of lookup price rules: Price book entries are not designed to handle attribute-based variations. Instead, lookup price rules should be used. These rules allow you to adjust pricing based on specific product attributes, providing more flexibility and accuracy1.
C: Creating process builders and flows to pass data between fields instead of using twin field mapping:
Process builders and flows can be complex and difficult to maintain. Instead, twin field mapping should be used. This feature allows you to map fields between objects, ensuring data consistency and reducing the need for custom code1.
D: Writing customizations for product selection or validation instead of using option constraints, product rules, and bundles: Customizations can be time-consuming and costly to implement and maintain. Instead, Salesforce CPQ provides built-in features like option constraints, product rules, and bundles that can handle product selection and validation. These features are designed to enforce business rules and ensure that only valid product combinations are selected1.
References:
https://inspireplanner.com/blog/5-common-salesforce-cpq-implementation-challenges-and-how-to-avoid-them/

In Salesforce Revenue Cloud, usage records are a key component of the usage-based pricing model. They track the consumption of a product or service over a predefined time period1.
The required fields on the usage record to load and rate the usage are:
* Start Date Time: This field represents the start of the usage period1.
* End Date Time: This field represents the end of the usage period1.
* Matching ID: Salesforce Billing uploads usage only to usage summaries with matching IDs1.
* Matching Attribute: Usages are rated against usage summaries where the Matching ID, Matching Attribute, and Unit of Measure fields all match1.
* Unit of Measure: This field represents the unit in which the usage is measured1.
* Quantity: This field represents the amount of usage1.
These fields are essential for accurately tracking and billing usage-based products or services. They allow Salesforce Revenue Cloud to calculate charges based on the actual consumption of a product or service, which can be more flexible and customer-friendly than flat-rate pricing2.
References
* Usage Summary and Usage Fields - Salesforce
* Usage-Based Products - Salesforce

Salesforce AppExchange offers a variety of solutions that can enhance the functionality of Salesforce Revenue Cloud. Here are the reasons why you would need an AppExchange solution to support generating a document in support of a Revenue Cloud project:
Watermarks (A): Watermarks can be used to mark documents asconfidential or to denote the status of the document (draft, approved, etc.). AppExchange solutions can provide this functionality, which is not available out-of-the-box in Salesforce.
Electronic Signature ©: Electronic signatures are often required for contracts and other legal documents.
AppExchange solutions like DocuSign can integrate with Salesforce to provide this functionality.
Contract Redlining (D): Contract redlining allows for tracking changes and comments during the contract negotiation process.This is a specialized functionality that can be provided by AppExchange solutions.
References
10 Benefits of Using Salesforce AppExchange Apps - Cloud Analogy
Revenue Cloud Collection Page - Salesforce
Extending Customer 360 with Salesforce AppExchange
What Is Salesforce AppExchange and How Can I Use It?

In Salesforce CPQ, the Order Management plugin functionality provides flexibility in customizing the default package behavior, including the abilityto set the activation date for orders. This feature allows for greater control over the order management process, enabling organizations to align the activation dates of their orders with their specific business requirements. By overriding the default behavior, businesses can ensure that the activation dates are set in a manner that is consistent with their operational workflows, contractual obligations, or customer agreements. This capability is part of Salesforce CPQ's broader set of features designed toprovide customizable solutions to complex quoting and order management challenges.