For modern DSLRs, the stated viewfinder coverage is generally understood as field-of-view coverage, not image area. In practice, that means the finder shows about 95% of the final frame dimensions, so the captured image will include a little more around the edges than you saw.

Whether you think of it as horizontal, vertical, or diagonal doesn’t usually matter for interpreting the spec: it’s a framing coverage figure, not an area percentage. If manufacturers quoted area instead, the number would be smaller, and makers generally prefer the larger, more marketable figure.

So a listed 95% viewfinder coverage should normally be read as approximately 95% of the frame view, not 95% of the total image area.