Occasionally it is necessary to convert an image profile from one to another. This can be done within Photoshop, when generating a PDF or even (depending on the printer driver) within print dialog boxes.
Since each color space includes a different gamut, there is a need to specify what to do with hues that don't exist in the new space. Should the closest available color be used even if it doesn't match? Should all colors be adjusted to simulate the relative differences in color?
Here's the best two options for photography: "Relative Colorimetric" preserves the relationship between gamut colors, even if this clips hues that are out of gamut. "Perceptual" rendering also attempts to preserve relationships even among out of gamut colors by compressing the entire tonal range without destroying any color information. The latter may produce smoother gradations when values are out of gamut.
Black Point Compensation is also a recommended option when converting between color profiles