Frigates are multipurpose early naval units that have light shore bombardment and anti-naval capabilities and also provide anti-aircraft fire. They also have sonar and radar and are important intelligence units. Certain factions versions have other abilities like the UEF frigate's jammer or the Aeon frigate's torpedo defense system, it should be noted that the Aeon version has two guns that can only both hit the same target if they are broadsides and lacks anti-aircraft abilities. These ships are even flimsier than cruisers, though, so they should be used for their supporting abilities later in the game, or to quickly and cheaply build a large navy.
Attack Submarines Edit
Submarines are anti-naval units and good at taking out ships especially ones lacking torpedoes. They can surface and submerge themselves underwater which makes them almost invisible in areas lacking sonar coverage. They also have some sonar abilities and all except the Aeon ship have a deck gun they can shoot when surfaced. The only other unique ability of a submarine is that the Seraphim ones have light torpedo defense systems.
Attack Boat Edit
The Attack Boat is a unit unique to the Aeon which carries their T1 naval anti-air defenses. It is fast and small but it cannot shoot at non-aerial units and require an escort to be kept safe. Though inconvenient in that you must build another necessary extra ship in an Aeon fleet this is somewhat countered in that its AA abilities are better than the systems mounted on other faction's frigates.