There are 2 hoops to get through at Delta. The first hoop is getting your app pulled and scored. The second hoop is then getting a high enough score on that review to warrant an interview offer.

An "internal rec", as I'll call it, gets you through hoop #1, guaranteed. It has absolutely nothing to do with airlineapps. Rather, a Delta pilot has a way to email the pilot recruiting team your name, and they will then review your app within 30 days. The nature of the email sent by the Delta pilot has no bearing whatsoever other than getting through hoop #1. It can simply state your name and maybe some other basic identifying information, or it can be the most flowery, glowing review ever written. All it does is get your app reviewed.

A "Letter of Recommendation" is an actual "letter" written by someone (anyone, Delta pilot or otherwise) that is electronically submitted and attached to your airlineapps application. This is the flowery "he's the best pilot/parent/volunteer/whatever blah blah stuff. These letters only assist in getting through hoop #2. To what degree, we don't really know, other than to make sure you meet the requirements posted on the letter of recommendation section (I think at least one person has to have known you at least 10 years??). Certainly having a handful from Delta pilots would probably look good, as well as others from other airlines and even outside aviation.

As you can see there is no connection between the two. A Delta pilot can write both an internal rec and a LoR, or just one or the other. As has been stated your app can only be scored once per year, so make sure you specify what you are asking a Delta pilot for, or else he/she may turn in an internal rec when all you wanted at the time was an LoR because your app wasn't ready for prime time yet.