Needed a derailleur on a build. Pulled a Shimano RD A550 from the parts box.
Seems to be working fine but then I noticed an extra screw on the back of it.
Some digging around, I found the instructions.
Seems the screw controls the “return force” of the RD and I should use a stronger return force if I’m riding in “poor conditions.”
I’m only ever going to ride friction, and I’m never going to ride those monstrosities called bifters, so why does return force matter?
Shimano data sheet follows.

Never seen that before. It sounds like its an older version of a modern derailleur with clutch. Setting the return force to high would help chain retention on a single chainring setup but probably not required on a double chainring setup unless you are going cyclo crossing.

The modern clutch is controlling chain tension only.

This screw adjusts the derailer's return force acting on the cable.

It is a fault of the design of slanted parallelogram derailers that causes the cage to bounce inward toward the larger cogs in response to bumpy roads taken at speed.
The higher return spring tension combats this, but makes for more shifting effort and friction/stress/elasticity going on with the cable, none of which is good for shifting effort or performance.

Recent 1X rear derailershave returned to non-slanted parallelogram designs, with only the cage pulley offset being used to track the conical/concave profile of the cassette.
This has allowed some lowering of the return spring tension, but the design can't really cope with the chain tension variations that occur when shifting chainrings.

Modern 2x and 3x rear derailers have dealt with the cable tension issue to some degree by using longer-travel, lower-rate return springs, which at least see less drop off of cable tension as the derailer moves toward the smallest cogs. This somewhat negates the issues with bounce and with the high spring tensions that used to occur as the derailer approached the largest cogs, but riding off-road can generate high enough chain sag forces that traditional slanted derailers may really struggle to control.

For downtube shifters, I leave it on light on my bikes. Strong is usually only used with STI shifters. You may find your front derailleur has this adjustment too.
Around this crossover era this adjuster was on several of the Shimano road product lines, 74xx derailleurs have it, also some of the 64xx series, and it appears 105x to0.

Also available on m952 XTR RDs.

Thanks. I will move this to light before the next ride.

I knew I had seen this screw on some derailleur before! If I still had a mountain bike I coulda just checked there.