When you select h.264+ or h.265+ the i-frame setting is greyed-out (set / managed automatically - you cannot set or change it).
In other CODECs (h.264/h.265 without the +):
They work to reduce bandwidth by only sending a full image initial / I frame once, then partial frames (P-frames) (containing only the changes in the scene since the initial 'I' frame.
So sending I-frames more often improves the video quality, but uses more bandwidth.
The I-frame interval in these HikVision cameras can be set between 1 and 400.
The lower the number, the smaller the interval between initial/I frames and the better the resultant video.
I never tend to adjust it.
h.265+ seems to work well and is very efficient, saving 70-80% bandwidth/storage over h.264.
BTW there is contextual Help available for all the HikVision Configuration pages (top right of each page, adjacent to Logout).
Sometimes it's truly helpful, other times (as in this case I suspect) it leaves you wondering what they are trying to tell you
Having more I frames doesn’t boost quality. It’ll speed up time to recover if there’s some kind of stream corruption as the decoder can only start on an I frame. Sometimes known as a key frame or IDR (instantaneous decoder refresh).
Some people like to have one every couple of seconds.