What is the purpose of plastic insert/plug in Deore FD-M590? My Deore FD-M590 is missing the instructions how it should be mounted and I would guess like any front derailleur, however what confuses me is little  pink (my eyes) insert/plug which I don't know what to do with it.



What is the purpose of this thing?
You can search Shimano's Technical Documents site for manuals.

Here's the manual for the M590 BB, crank and front derailleur: https://si.shimano.com/pdfs/si/SI-6SA0A-001-ENG.pdf

The red insert is apparently a 'Pro-Set Alignment Block' and is there to help set the height of the cage above the large chainring.