Colony Ships and Mining Ships are purchased using the TREASURY.
Military Ships, Space Ports and Construction of Anchorages are purchased using your MILITARY COFFER.
the only way to add money to your MILITARY COFFER, is with Military Tax Reinvestment, or by selling military ships.
All Defense Turrets, and also the Singularity Excimer Power Plant are purchased using the MILITARY COFFER. All the rest is purchased using your TREASURY.