we are working in a large font with plenty of masters and axes, including Grade and ROnd. Those masters metrics are synced with the sharp masters using the appropiate custom parameter.
As master Rainer says we try to have all the glyphs with components autoaligned, but for some cases like the lslash, we use autoalignment + ==XX in the sidebearings to get some extra space. Those values do not propagate to the synced masters automatically, so we have to enter them manually
The sync metrics are do sync the width and the kerning. Metrics keys might be useful in this particular case but probably would do more harm quite often.
my point is that I would say that the custom parameter should propagate the metric keys to other synced masters no? I don t think it makes sense to have a command that specifies that general order, and then have another command that instead of complementing (adding additional instructions) overrides the main command.