Link to Mod (http://custom.simplemachines.org/mods/index.php?mod=3024)
When members are put in non-default primary groups, then their quota is always taken from the quota settings of those primary groups, and the quota settings specified in groups based on post count are ignored. This mod ensures that PM quotas are always based on post count based group limit settings.
I don't get it, if I install this, what will it do exactly?
The description should be clear enough. Basically, this allows you to enforce different PM quotas for various post count based groups, even when you put people in various primary groups that you have created.
Usually when you create your own groups and make those groups the primary groups for members, then their PM quota is based on the quota you specify for those newly created primary groups, and any post-count based PM quota is ignored. This mod prevents this from happening, allowing you to give more quota to people who have higher post count, while allowing different primary groups for them.