The C18 Star Lifter Air Transport is an UEF air unit. Its is primarily a transport, capable of holding up to 15 units or 25 with research. But it can't transport any experimentals. It has two AA Linked Railguns: one at the front and one at the back. It can also gain shields through research. While it can be easily shot down by 5-6 AA turrets, the C-18 can hold its own against a couple of fighters. While it can carry ACUs, this is extremely risky because if the relatively weak transport is destroyed, so is the ACU.
When a transport is destroyed, all units it was carrying are also destroyed. This is a much greater loss of resources than just the transport's construction cost. It may not be possible to protect all the transports as they deliver units to a contested zone, so filling all transports with units guarantees the maximum loss of resources. Instead, mix some empty transports into your transport group. Your opponent cannot tell which transports are loaded, so at least part of the anti-air defenses will only destroy the relatively cheap transports.
As they are relatively fast they are good to transport your units close to the enemy base. But unlike the C-230 Star King Extreme Experimental Air Transport, it can be shot down very very easy especially if the enemy researched the Linked Railgun Anti-Air (available for mass extractors and research stations) so ALWAYS unload the units nearby the enemy base away from the range of the AA guns not in the enemy base because you might not have any transport in one piece.
Another way of using the transport without having it be shot down ever so easily, is to nuke the area where you want to transport your troops. This will clear the area of any AA turrets, and will give you a nice open space to place your troops. Note: DO NOT fire a nuke in the middle of the enemy's base for this strategy because while there is a cleared area in the middle, the outskirts will still have functional AA turrets. Another good idea is to already have units present in the cleared area(for example: fighters and gunships) to cover the transports and the newly deployed units.