This reports the amount of free frame-buffer memory remaining on this card, in bytes, the byte-count counterpart to the percentage figure in cardFrameMemoryUtil. Because it's expressed in absolute bytes rather than percent, this is what an admin uses when they need to know the actual headroom in real terms, for example to judge whether a specific burst size would exhaust the buffer, rather than just a relative fill level. For example, cardFrameMemoryUsage showing only 4KB free explains why even a modest traffic burst on that card starts triggering discards.