Fuel and ammo
Aircraft can't fly around forever and Spring:1944 understands that. Any air support you call in will have a limited time, during which you can boss them around.
Fuel time can be shortened if the enemy deals damage to the plane. This is usually proportonal to the fear damage of the weapon.
When an aircraft runs out of fuel, it will fly off the map, preferebly through the side they arrived by. When retreating, aircraft will return at most 65% of the command spent on it's call in. This amount decreases with remaining health, so if you have only have half of the health remaining on the aircraft, you will get 0.65*0.5 = 0.325 of the amount spent on the sortie.
Some units require ammo to fire. Ammo can be seen as the yellow logistics bar.
Near the bar you will find a countdown timer. When the countdown reaches 0, the logistics bar will refill completely. To get more logistics refilled, you need to build supply depots. Making more supply depots is very expencive and destroying them is usually pretty easy, as they are filled with explosives, so make balanced decidions and don't make supply that you won't need. More on supply in Economy
Units carry a limited amount of ammunition with them, which can be looked up in the tooltip. Current ammo can be seen as the yellow bar below the unit.
To rearm itself, said unit needs to be inside a yellow rearm circle. To see these, select any building or unit, capable or rearming others. These include: any production facilities (excluding air stations), deployed supply trucks and halftrack transports.
Even if an aircraft uses ammunition (like bombs or missiles), it cannot be rearmed. If a bomber says it has one bomb then that's all you got to work with. On the flip side, aircraft that do use ammunition don't drain you logistics store. As a result, air units do not interact with your supply of ammunition (think of it as them draining ammunition from an airfield base).
Fire rate bonus
While inside a rearming circle, a unit will gain a neat +15% to it's fire rate, no matter if the unit actually requires ammo to fire. There is but one condition for this to work - you must have at least 100 supply still unused. You can see if a unit is getting this bonus or not by looking for an icon below it a yellow circle with a plus will mean the bonus is applied, a grey one will mean the unit is in a resupply circle, but you don't have enough unspent ammo for the bunos to work.