Relative Colorimetric rendering uses a different mapping for contrast and crhomacity values.
The contrast (brightness channel) is mapped to the target color space, and no loss is seen in the image highlight or shadow detail areas.
All colors (chromacity) who can be reproduced in both color spaces, will keep their original saturation and hue.
This will result in a more saturated image.
Colors outside the targets color space, are ‘clipped’ to the first reproducable color in the target color space. This can result in a loss of detail in the high saturated parts of the image. The reproduction will normally match the original more closely then when Perceptual rendering is used.
So, Relative Colorimetric is a great rendering intent, but it can be dangerous when useing a very big original color space and a relatively small target color space.
When using Relative Colorimetric rendering in Adobe Photoshop, we also have to choose a setting for the Blackpoint compensation.
When turned on, the neutral blackpoint from the original color space, will be mapped towards the neutral blackpoint of the target color space, the same way it happens when using Perceptual rendering.
Shadow details are kept when using blackpoint compensation.
When turned off, no mapping for the blackpoints is performed which will (in case of RGB to CMYK rendering) result in a loss of shadow detail.