Can someone explain the seating options on Spain's AVE?

The letter codes are different fares. The onlyone that affects seating is the M fare which is for 4 people sitting around a table, facing each other. The other fares are Promo, Promo Plus, and Flexible fares in regular seats. If you chose a Promo (P)fare in Turista class, you two will be seated side by side in two seats facing the backs of the seats in front of you. Not facing two strangers across a table. The only downside to P fares is they are not changeable and non-refundable. Promo Plus fares allow a change for a fee.

Seating in turista is 2 aisle 2 and in preference or first class the seating is 2 aisle 1. Often times the table for four is only available in the middle of the car. Most of the time the seats all face the middle of the car or another way is that half of the seats are riding backwards that way the car can run in either direction. Sometimes you will find clusters of seats facing each other. It is hard to know the exact configuration of the car.

The AVE trains we rode had some seats facing the car ends and some facing the middle (where there were tables). Buying tickets in advance on Renfe, we were never assigned seats riding backwards. I seem to recall being offered a chance to choose seats on at least one of the trains, but I can't confirm that because the offer does not appear until after you enter credit card details.

OK, I found a seat map, thanks to the Man in Seat 61: http://www.seat61.com/train-seat-plans.htm#Italy It says Italy but you can scroll down to Spain.

Many thanks to everyone. I was able to view the seating chart and get a full understanding of the prices. Now, if I could have gotten Renfe to cooperate. I tried, to no end, to get my credits cards to work on the website. After hours of attempts I finally emailed our friends in Spain and they purchsed our train tickets for us. I tried Renfe's English version and then the Spanish version - no luck. I checked with my banks twice to confirm that the cards would work overseas which lead me to believe that Renfe website was the culprit.

Dominic, the Renfe site is notorious for not taking American credit cards, unless they are registered with "Verified by Visa." Once I did that I was able to buy tickets on Renfe with no problem. But apparently not all credit cards can be registered. An alternative is the Spanish agency Petrabax, which offers the discount prices but adds a small handling fee. I know you have your tickets, but I am mentioning this for the benefits of others who micht want to try buying on Renfe.

at the ends of the coaches are "4-seaters" that is 2 seats facing 2 seats, with a table in the middle. They are sold as a package. You can buy the 4 and spread out, otherwise you won't be in them.