Google says
These reducers are used to reduce the line capacity of a larger Shimano reel (like the 14000) to a smaller size, such as 6000.
How they work:
They are designed to clip onto the spool and reduce the line capacity to the Shimano stated 6000 capacity.
Reels they fit:
The Shimano 6000 spool reducer fits reels like the Aero Technium Magnesium 14000 XTC/XSC, Power Aero 14000 XTB/XSB, and Ultegra CI4 Plus 14000 XTC/XSC/XTB/XSB.
Other sizes:
Shimano also offers spool reducers in 10000 and 3500 sizes.
Line capacity:
The 6000 reducer will allow you to spool a 6000 capacity, which is typically around 300 yards of 10lb line.
Examples:
You can get about 275 yards of 0.33 mono line on a 6000 reducer.
Some users have successfully spooled 250 yards of 0.40 mono line with the 6000 reducer.
With a 6000 reducer, you can spool 190m of 0.40mm line without any backing.
I did t know how they worked but it now makes sense
you can find the line capacity of a 6000 reel or 10000 reel to see what they hold and that’s close enough from the look of it