Salesforce Sales Cloud is the system of record for the sales process because it contains information about opportunities and opportunity products, which are used for forecasting and reporting. It also integrates with other clouds such as B2B Commerce and CPQ to support the cart-to-quote process.
https://help.salesforce.com/s/articleView?id=icx_b2c_crosscloudengagement_systems_of_record.htm&language=en_US&type=5

B2B Commerce is a solution that enables B2B buyers to purchase products online from a branded storefront. It can help Northern Trail Hot Tubs provide custom pricing and configuration options for their Hot Tubs without having to wait for estimates. It can also help them track and manage their orders and inventory1.
Service Cloud is a solution that enables customer service agents to manage and resolve customer issues across multiple channels. It can help Northern Trail Hot Tubs support their Dealers and Customers directly and provide them with visibility into their case history and status2. It can also help them leverage AI and automation to improve service efficiency and quality.
By integrating B2B Commerce and Service Cloud, Northern Trail Hot Tubs can create a seamless B2B2C experience that enhances customer satisfaction and loyalty3

The best three leading metrics to help NTO measure each reseller's goals through the Partner Community are logins into Partner Community, number of quotes generated, and opportunities generated. These metrics will give NTO early feedback on how the portal is being used by their partners and will provide insight into their success in using the Partner Community. Product types sold and opportunity win rates are lagging metrics and may not provide timely feedback on the success of the Partner Community.
Leading metrics are indicators that show what's happening and can have real-time impact on your bottom line12.
Lagging metrics are indicators that show the outcome of what happened in a previous time period12.
Leading metrics are useful for predicting future performance and making adjustments, while lagging metrics are useful for evaluating past performance and setting goals34.

According to 1, a single source of truth (SSOT) is the practice of aggregating the data from many systems within an organization to a single location. A SSOT is not a system, tool, or strategy, but rather a state of being for a company's data in that it can all be found via a single reference point.
According to 2, 3 and 4, Salesforce has introduced the Customer 360 Truth Platform, which includes MuleSoft. This is a new set of data and identity services that enable companies to build a single source of truth across all of their customer relationships. This connects data from all Salesforce clouds to create a single view of the customer.
MuleSoft Anypoint Platform is an integration platform that allows companies to connect any application, data source or device using APIs. MuleSoft enables companies to leverage their existing systems and data sources without having to replace them or create custom integrations.

According to 1, Partner Communities have access to the full sharing model, while Customer Communities can only access Sharing Sets and Sharing Groups. This means that you can use different sharing settings for your internal and external users, depending on their roles and profiles.
The external sharing model allows you to specify a default level of access for each object for your external users. You can also use criteria-based sharing rules, manual sharing, Apex managed sharing, and implicit sharing23 to grant additional access as needed.