Here is a useful report regarding the interpretation, quality assessment, and handling of this file type.

If you are a developer building custom ROMs for an ARM32 A‑only device:

: This indicates the file is compressed using XZ compression to save bandwidth during download. You must decompress this before flashing. What is "Extra Quality" in This Context?

The term likely derives from:

If you have more context about where you encountered this string, I could provide a more accurate interpretation.

# Maximum compression (trade-off speed for size) xz -9 -k -e system.img