The icon order is determined by the contents of /usr/local/cpanel/base/frontend/x3/dynamicui.conf. There are some rudimentary comments within the file, to group the sections. Modifying the itemorder= entry on a line changes where the icon appears within the group. Modifying the grouporder= entry on a line changes order of the groups (or sections)
When modifying this file please keep the following in mind:
1. This file, if modified, will be replaced during updates.
2. You may need to search your system for other copies of dynamicui.conf if your itemorder is not being observed
3. There's no telling what could happen upon modifying this file. You could have all users everywhere singing your praises for improved icon ordering, or puppies and kittens everywhere could be forever sad.