E-Certificate or Promo code on United
 

Whenever I visit UA.COM, I see a field for:

E-Certificate or
Promotion Code

How does one go about getting these? Are these targeted mailings?

E-Certificates are often the result of promos, though I believe you can also use a ticket's residual value here. Promotion Codes are typically for conventions.

You can also plug in a promotion code from current united promotions. Example, if UA is offering double miles on certain routes, but only at certain fares, if you plug in the code (available by looking at the promotions list online), you will get eligible flights.

This does not mean that ual.com will cooperate with actually booking you into the flights! One of the bugs of the system.