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For SD content, you are starting out with a low quality source. If you want to refrain from degrading much further, you use a lower RF.
For HD content, you could apply the same logic as above. I.e. "you want to refrain from degrading it much further". But this is overkill for the vast majority of people. The original quality of HD sources is often more than adequate for most peoples viewing environments and it is OK to allow a bit more loss during the encoding process in order to achieve smaller file sizes.
Additionally, SD sources are typically viewed on HD or higher displays, magnifying imperfections. So it's necessary to use a lower RF to avoid introducing artifacts that will be more noticeable in this case.
Rate Factor is not an absolute quality metric. It's more like quality-per-pixel. So the output resolution is an important factor in what RF range is acceptable.