In a recent session of the Alien RPG, players asked how much artillery the USCM could call down if needed.
In general, the USCM make significant use of tactical computer systems to allow even small units to direct artillery strikes without consulting the chain of command.
Each section has an APC which is equipped with 51 mm DSGR laser-guided precision strike 'smart' missiles. These have kinetic warheads, so there is no blast effect. Accuracy is dependent on the target designator, but a skilled operator at reasonable range should be able to put them into a 100 mm radius target. They should hit about as hard as an autocannon round, so plenty to kill a person or xenomorph, and threaten lightly armoured vehicles.
Each platoon has a single 80mm mortar, which would usually be directed from the tactical operations centre of an APC. Lethal blast radius should be about 12 metres (on the tabletop this would be about 8" in ~1:60 scale, so picture slapping a 16" pizza on the board), with fragmentation possibly extending casualties out to about twice that. Accuracy is good enough to ensure that the target is somewhere within the blast radius almost every time.
The UD-4 dropship which is often attached to a platoon carries 32 Banshee 70 rockets. Each of these is capable of causing casualties over an area of over half a hectare (or one American football field), about three times the radius of the 80 mm mortar round. Repeated strikes would be necessary to ensure everything in the area was disabled, especially if they were armoured, inside buildings, or super-humanly tough.
At the company level, a special weapons platoon adds another mortar and APC. Mortar carriers from an artillery company can be attached to infantry companies, which increases the number of shells that can be put into the air, but doesn't otherwise extend the unit's capabilities.
Above the company level is the dedicated artillery company which is typically attached to an infantry battalion. The main artillery piece at this level is the M292 Self-Propelled Gun, a large tracked vehicle. It is armed with a 158mm gun, capable of shelling targets up to 62 km away. Some of the shells it uses are homing weapons, allowing for precise bombardment. Maximum effect may be similar to that of a Banshee 70 rocket. An even heavier option is the M201 Multiple Rocket Launch System, which has a range of 120 km and is described as 'accurate', which probably means it also has guided or homing capabilities. This should affect an area about four times that of a Banshee 70 rocket (i.e. twice the radius).
Spacecraft can deliver nuclear weapons to the surface of a planet via re-entry vehicles or cruise missiles. Maximum yield is one MT TNT-equivalent (causing casualties in a radius of around 8 km, although more-likely-than-not fatalities would occur at only about half that distance). Use of nuclear weapons requires approval of the most senior officer in the theatre of operations, but there doesn't seem to be a lower limit to that (e.g. in Aliens, corporal Hicks seemed able to do it while Lt. Gorman was incapacitated). There are also 'strict guidelines' on the use of nuclear weapons on inhabited planets.
It isn't entirely clear whether the nuclear weapons used by the USCM are thermonuclear fission-ignited fusion ones (like modern-day ones) or 'pure' fusion ignited by antimatter, superconducting z-pinch device, hafnium isomer explosives, or some other science-fiction technology. The latter would be far safer and more flexible, but the significant caution about deploying them suggests that they are actually not that safe. Pure fusion devices such as atmosphere processors and spacecraft engines do exist in the Aliens setting, but it is possible that there is a minimum size which makes them impractical as (tactical-scale) weapons.
