One possible reason why a new custom field is not available in the data stream is that the Salesforce Integration User, which is used to connect to the CRM source, does not have "Read" permissions on that field2. To fix this issue, the administrator needs to grant the appropriate permissions to the Integration User profile or permission set.

The Salesforce CRM connector supports ingestion of standard objects and custom objects from Salesforce CRM. Standard objects are predefined objects that are included with Salesforce CRM, such as Account, Contact, Lead, etc. Custom objects are objects that customers can create to store information that is specific to their organization or industry. Reference: [Salesforce CRM Connector], [Standard Objects], [Custom Objects]

This is the first thing a business stakeholder should focus on when considering a Data Cloud implementation. Obtaining cross-organizational buy-in involves aligning the goals and expectations of different teams and stakeholders who will use or contribute to Data Cloud. Reference: https://trailhead.salesforce.com/content/learn/modules/customer-data-platform-basics/get-started-with-customer-data-platform

COALESCE is the function that returns the first value from a list that isn't empty. It takes a list of values as arguments and returns the first non-empty value. For example, COALESCE(NULL, '', 'Hello', 'World') returns 'Hello'. If all values are empty, it returns NULL. Reference: COALESCE

The Source Sequence reconciliation rule allows you to sort your data sources in order of most to least preferred for inclusion in a unified profile9. For example, you can specify that data from Marketing Cloud should have higher priority than data from Service Cloud when selecting attributes such as first name or last name.