In Spain, more precisely in the Basque Country, I have seen letters "R" and "E" before parking slots.
What do they mean / stand for?
This one is in the town of Zumaia. Picture is taken on Google Street View.
According to the Zumaia website:
There will be 3 parking areas:
- Resident area (painted white with the letters R and E)
- Areas of high rotation (A, B and C: painted blue and red)
- Half-stay area (painted blue)
(Google translation, emphasis mine).
Note that this seems to be in use in some other places. Other cities make extensive use of blue-and-red, blue-and-green and whatnot.
Just to complete the consequences of those innocent letters:
Only people with a resident card can park here.