Originally Posted by
xbartx
...I don't see it having to do with the trim on the front derailleur because it doesn't involve shifting from one chain ring to another, it is that the chain touches the inner part of the derailleur cage when the rear cassette is on the largest gear and with the same front derailleur position when I shift the rear to the smallest gear the chain touches the outer part of the derailleur cage...
That is what the front trim function does.
https://si.shimano.com/pdfs/dm/DM-RAFD001-04-ENG.pdf
Front derailleur how-to: shifting and trim