My camera is in Auto Focus mode (AF). As my understanding, the camera will focus on subject automatically. But when I rotate the focus ring, the focus subject points will change, too. I don't understand why. As my understanding, Focus ring is only used when Manual focus (MF) used.
If I rotate focus ring in AF mode, does this hurt the camera or lens? My lens is Sony DT 18-135mm F3.5-5.6 SAM Zoom Lens. ( The User guide of the lens does not mention about AF & rotating focus ring issue).
Focus will obviously change if you rotate the focus ring. There is no exception to that.
Now, some lenses allow manual focus override even in Auto-mode, in Nikon world these are indicated with A-M (Auto with Manual override) on the lens. If your lens mechanics do not support this, you are working against the focus motor when you change focus manually in AF mode.
Also, some lenses do indicate A-M even in the User Manual (e.g. a 18-105mm Nikkor), but I noticed that although Manual focus in Auto mode is possible, it made my lens much noiser after I tried this...
Back to your specific lens: from photo.net:
SSM – Super Sonic Motor. Lenses employing SSM feature quiet, fast, smooth, and accurate focusing. All of Sony’s SSM lenses also feature direct manual focus override, allowing a photographer to fine-tune focusing without using the AF/MF switch by simply turning the focus ring.
SAM – Smooth Autofocus Motor. Lenses using a SAM focusing system feature smooth and accurate focusing. SAM is not equivalent to SSM, and does not have a clutch in the focus ring to enable DMF override.
DMF being: direct manual focus.
So as it seems, your lens does not support this. It means that if AF is on and is attempting to move focus and meanwhile you move or hold the focus ring, you are working against the motor. Whether you will damage the motor or dislocate something in the lens, no clue.
Don't use AF focusing and focus ring the same time. I would actually avoid touching the focus ring unless in MF mode.
To add TFuto's answer, it also depends on whether the camera/lens uses a digital focus ring, which isn't directly connected to the lens/motor. In that case the ring won't move during AF and you can't hurt anything by rotating it.