Let me see if I can explain it for you, in a way it doesn’t even make sense to me since the resolution dimensions in C4D are in pixels and then you are given the option of adding more in DPI or PPI, dots or pixels per inch.
If you render an image in C4D at 800×600 in 600 DPI, then open it in photoshop and enlarge to 1600×1200 with resampling off, the image would then be 300 dpi at 1600×1200.
If you just increase the DPI in photoshop to, lets say 300dpi on an image that was rendered at 72 dpi (screen resolution), you are increasing the resolution by interpolating and will get a poor result. Doing so in this way, the computer is sort of “guessing” what the added pixels should be. You can experiment on your own to see the results.