LinkedIn announced that it has made custom fields searchable in LinkedIn Recruiter. It is mainly more useful for finding people based on very specific type.
If a candidate who is not available up to a certain date, and a recruiter that uses the custom field search to find him based on the noted date.
To access this feature in a cases, you would open LinkedIn Recruiter’s advanced search page, enter a date range, and in the “Availability Date” custom field, the candidate’s profile would appear.