Not a dumb question at all! "R" is the fare bucket that upgrades come from, at least on domestic flights. If you see "R", you can get an upgrade with miles. I honestly don't know if "R" availability is required to get an EUA (I think so, but sheesh, I should know that...).

Added: Fare Buckets from this thread.


To sum up...
(150% Miles, 150% EQM)
A – Full Fare First
C - Business on Caribbean, Latin American, Pacifc markets with no BF cabin
D – First, First with International Connection
J - Full Fare BusinessFirst
Z - Discounted First or BusinessFirst or Business

(100% Miles, 150% EQM)
Y - Full Fare Coach
AU - Y Fare that has been upgraded at time of ticketing
H - Discounted Full Fare Coach

(100% Miles, 100% EQM)
K - Coach
N - Coach
B - Coach
O - Discounted Coach
V - Discounted Coach
U - Discounted Coach

(100% Miles, 50% EQM)
Q - Discounted Coach
I - Deep Discounted Coach
S - Deep Discounted Coach
W - Deep Discounted Coach
T - Deep Discounted Coach
X - Deep Deep Discounted Coach
L - Deep Deep Discounted Coach

** S, T, L are discounted coach itineraries that include connections. They are not used for non-stop itins from EWR, IAH, CLE**
** I, W, X are only available on CO metal, not codeshares**


(Upgrades)
AU - Y Fare that has been upgraded at time of ticketing, earns 150% EQMs
R - Elite First Upgrade - Milage and EUA

(Freebies)
E - First, Business, BusinessFirst Reward
EG – Platinum BF Reward
F - Platinum Elite BusinessFirst Reward / special BF Reward (e.g. increased availability for HNL/GUM/Asia residents)
M – Elite Coach Reward
G - Non-Elite / Elite Coach Reward or Bump Cert, Coach Certificate (Fly 3…)

(EazyPass)
YN – economy
P – First/BusinessFirst