How to do more than 1 adjustment within a partial selection.
Simplest example is when one has painted out, say a distracting notice with colour-picked colorize, then wants to use Filter>add grain/noise - arguably easier to go elsewhere and use clone tool, but clones that look OK on screen can jump out on print. Also useful when one doesn't know what one wants to do without trying 2 or 3 tools but within the same partial selection.

Have at last found 'Linked enhancements' on p68 0f my paper manual: just press shift key when summoning up 2nd and subsequent enhancements. Each then works within a 'partly selected area' and can of course be turned on/off individually.