It's the difference between point harmonization and pattern harmonization.
Point harmonization has all guns set to the same convergence, so that they all hit the same area at the same range. The word "same" is used with some artistic license though, since rapid firing guns on a vibrating, fast moving platform shooting at another such platform don't really manage to concentrate all rounds on one place, but you get the idea: point harmonization is for putting as much amount of lead as possible on the smallest target surface possible, to ensure maximum damage in the event of a hit.
Pattern harmonization is different, because the goal is to ensure maximum possibility of as much guns scoring a hit, at the expense of some damage in the event that the hit does occur because not all guns are harmonized for that particular hit and some will miss. In other words, if you are any good with aiming go for point harmonization, if you are struggling with placing rounds on target go for pattern harmonization. You'll do less damage when you score hits (since not all guns will be scoring them), but you'll be able to score them more often/easier.
Pattern harmonization is achieved by setting different convergence for each pair of guns and as such, it is better suited to allied fighters that tend to use banks of similar machine guns with a relatively high rate of fire (4,6 or even 8 of them) and not a 109 with it's 3-4 separate guns (depending on variant) some of which fire slower than the rest.
The final effect is that at different ranges, each pair of guns has a varying possibility of scoring a hit and an associated potential for damage if the hit is scored. The process is somewhat complicated to figure out manually and the best method is to dig up some historical documentation on where to have each pair of guns converge. I remember there was one such document, either on this forum or on the previous IL2 series section, detailing how it was set for US fighters with six machine guns in the pacific.
However, once you do find the needed information it's true that you don't need a mod to use it: we can already assign different convergence settings to each individual gun through the loadout options.
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