We get lots of questions about these sections for United and Delta.  There are three specific ways that a reference or a recommendation can be made.

General References:
These are found in the blue menu of airline apps but show in the specific addendum sections.  They can be customized by the applicant for each airline, and are what I would call static.  Delta or United might reach out to them to verify you, but likely only after a CJO has been made.

Professional Recommendations:
These are commonly referred to as letters of recommendation.  The applicant initiates them from the addendum section of the application and are airline specific.  Follow the Professional Recommendations link in each airlines addendum, and then enter the email for whom you are wanting to invite to write a letter on your behalf.   Any recommendation that is written in response to the email is now ‘attached’ to your application and can be seen by both you, and the airline.  The data is not editable for typos or content, but it can be deleted by the applicant, not the airline.  If you are not sure of what the form looks like, send one to yourself by putting in your email address.

Internal Recommendation:
Unique to Delta, this is the process by which an existing Delta employee contacts pilot hiring via a specific email address and recommends that Delta score your application.  Delta tells their folks that they will score the app within four weeks from receipt of that internal email.  As far as I know, no communication from Delta back to the employee is offered, the app is simply scored and placed in the queue accordingly.