It increases your market capability.  So if your market allows you to send 5m in rss to a clansman at lv 16 VIP, +50%, you can now send 7.5m in rss.

That thing is like u get some extra bags and bigger convoy size to fill ur rss and send to ur clansmen which is kinda good thing for bank and other players who need to send very often to others in big no. as sending 5mil at a time when other needs 200mil rss is bit tiresome so they have added this new feature and that convoy size can also be increased by knowledge in economic part. Convoy capacity 1 and 2 can be done till lvl5 which collectively will give u like 66% in increase so that is a huge bonus but also demands bigger rss to do it but after lvl5 it is insane.