Page layout and field level security are two factors that determine whether a user can see a new field on a record. To troubleshoot this issue, the administrator should ensure that the page layout for the user's profile has been updated to include the new field and that the field level security of the field for the user profile allows read or edit access. References:
https://help.salesforce.com/s/articleView?id=sf.customize_layoutoverview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.admin_fls.htm&type=5

Section: Volume C

To prevent users from selecting more than one value on a picklist field, the administrator needs to change the field type from a multi-select picklist to a regular picklist, and select the "Choose only one value" checkbox on the field definition page. This will ensure that only one value can be selected on the record page and in reports and filters. References: https://help.salesforce.com/s/articleView?id=sf.customize_picklists.htm&type=5

Section: Volume A
Explanation/Reference: