This can be done when it's used in custom centers (Customization > Centers and Tabs > Centers > New).
1. Creating a custom center tab with Center = All will insert it in the middle of Activities and Transactions tabs.
2. Creating a custom center tab with Center = Classic inserts it in the middle of Setup and Support tabs.
3. The arrangement of custom tabs depend on the time of creation. Those custom tabs between Activities and Transactions have the most recent ones beside Activities.
4. Those custom tabs between Setup and Support have the most recent ones beside Support.5. The arrangement of tabs depend on Center right upon creation.
Note: If a custom center tab AAA was originally assigned with Center = All, it will be located between Activities and Transactions. Changing its Center to Classic does not move it between Setup and Support.