Roughly, BPG [1] is for HEVC as what WebP is for VP8 or VP9, in the sense that you take an I-frame out of a video format and stuff it into an established, classic image container.

Compared to this, HEIF is a different way of packaging HEVC frames; namely into ISOBMFF (MPEG-4 Part 12), the most basic level of MPEG container. This comes with benefits [2] that come along with using that particular container, but also drawbacks, because with some of the advanced features of HEIF you're essentially describing a sequence of frames -- which is almost like video -- but you're doing it in a way incompatible with the way you'd describe video. I'd be curious if the two "representations" are losslessly convertible, for example.

[1] https://bellard.org/bpg/ [2] https://nokiatech.github.io/heif/technical.html