My Araya GP-710 rims have a sticker that says 'safety dot' with arrows that point to little dimples in the rim -what are they for ? Dimple not visible in pic, but are on the rim
As long as you can see the dimple, the rim is still thick enough.
When you brake, the rim gets worn. Eventually it is worn so thin that it can break. The dimples help you identify the wear so that you can replace the rim before it breaks.