Two reasons that would prevent a user from being deactivated are that the user is part of a territory hierarchy or that the user is assigned in workflow email alert. A territory hierarchy is a structure that defines how territories are related to each other in Salesforce; if a user is part of a territory hierarchy, they cannot be deactivated until they are removed from all territories. A workflow email alert is an action that sends an email to one or more recipients when a workflow rule is triggered; if a user is assigned in workflow email alert, they cannot be deactivated until they are removed from all email alerts. The user being in a custom hierarchy field or being the highest role in role hierarchy are not reasons that would prevent deactivation; they may affect data visibility or record ownership after deactivation, but they do not block deactivation itself. Reference: https://help.salesforce.com/s/articleView?id=sf.users_deactivate_considerations.htm&type=5

Standard objects are objects that are included with Salesforce by default, such as Account, Contact, Lead, Opportunity, etc. They have predefined fields and functionality that support common business processes. Custom objects are objects that you create to store information that is specific to your organization or industry. You can create new standard objects using the Object Manager in Setup. Reference: https://trailhead.salesforce.com/en/content/learn/modules/data_modeling/standard_and_custom_objects

Web-to-Case allows you to create cases from a form on your website.

An external ID is a custom field that has the external ID attribute enabled, which means it can be used as an identifier when importing and updating records from an external system. It allows administrators to match records based on a unique ID value from another system instead of using Salesforce record IDs, which may not be available or consistent across systems. An auto-number field is a custom field that automatically assigns a unique numeric value to each record, but it cannot be used as an identifier when importing and updating records from an external system because it is generated by Salesforce and may not match with the external system's IDs. A rich text field is a custom field that allows users to enter formatted text, images, and links, but it cannot be used as an identifier when importing and updating records from an external system because it is not unique or consistent across systems. A record ID is an internal ID assigned by Salesforce to each record, but it cannot be used as an identifier when importing and updating records from an external system because it may not be available or consistent across systems. Reference: https://help.salesforce.com/s/articleView?id=sf.custom_field_attributes.htm&type=5

To display only the proper lead sources based on the product category chosen, an administrator should create a dependency between the Product Category field and Lead Source field on Lead object. A dependency is a relationship between two picklist fields that restricts the values available in one picklist based on the value selected in another picklist. For example, an administrator can create a dependency that shows only Online Store and Social Media as lead sources if Product Category is Shoes, but shows only Trade Show and Magazine as lead sources if Product Category is Apparel. Creating a page layout for each category, creating business processes and record types for each category, or creating a single business process with record types for each category will not display only the proper lead sources based on the product category chosen. Reference: https://help.salesforce.com/s/articleView?id=sf.customize_dependent.htm&type=5