What do letters "R" and "E" before parking slots in Spain mean?

Question

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.

Answer 1

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:

Resident area:

Only people with a resident card can park here.