Generally speaking, the three tier levels for combat operations:
1. Infantry (ground troops, the folks who get shot at and face the demon as a basic job requirement)
2. Command (REMF [for those who don’t know, Rear Echelon Mother Fs] tacticians, the same tacticians who say which Infantry unit faces the demon and on which day the face-off is supposed to happen)
3. support (anything that can be immediately delivered to get the demons off the Infantry’s butt, even if that delivery is just a logistical resupply of [preferably “dry”] TP)

In reality, you can even have tier levels within tier levels, such as within the above Tier 1 you will find:
• Tier 1. Fire Teams A & B (Individual Infantry Soldiers from A Co, 1st Platoon, 3rd Squad).
• Tier 2. Light Artillery (A Co, Weapons Platoon).
• Tier 3. Heavy Artillery (29th Field Artillery).
• Tier 4. Air Support (medivac, suppression, supply-drop, etc...).

Example:
The system ONR is demonstrating is called WARNET-the Wide Area Relay Network. WARNET is a system-of-systems organized into three tiers. Tier 1 connects small dismounted infantry units and vehicles on the ground, Tier 2 links unit headquarters ashore and afloat, and Tier 3 provides the airborne communications network that enables over-the-horizon connectivity throughout the battlespace. The entire network is wireless, fast, robust, and high-bandwidth. It connects users over the horizon by using airborne relays as opposed to scarce satellite resources. The major components participating in the demonstration include:



• Upgrades to the Tactical Common Data Link (TCDL) High Data Rate Point-to-Point. TCDL is a high data rate microwave radio. ELB has upgraded it to give it an air-to-air capability.
• Upgrades to the AN/VRC-99A tactical radio. ELB has upgraded this radio to give it an adaptive burst rate and dynamic time slot allocation.
• Upgrades to the RT-1812 tactical radio (formerly the Near Term Digital Radio-NTDR). ELB has added a multicast capability to this radio.
• End User Terminal. A situational awareness and networking tool for the small unit leader.
• WaveLAN II. A commercial off-the-shelf local area network card, to which ELB has added an amplifier for improved military performance.
• SECNET 11. This wireless Ethernet card has been modified to include levels of security appropriate to military expeditionary operations.
• Network Status Panel. A real-time tool for monitoring network connectivity in a mobile tactical environment.
• Battlefield Visualization Tool. Software that can be installed in a commercial laptop or palmtop computer. It gives the Marine on the ground access to information-reconnaissance imagery, friendly and enemy unit positions, etc.-normally found only on headquarters maps.
• Middleware tools for fast, flexible, and cost-effective interoperability. Information Broker and the Marine Corps Systems Interface Translator enable a wide array of different legacy and emerging platforms to operate together seamlessly.


(Source)

In a tactical reality, tier level would have to depend upon what is being discussed. For instance, just as the job force of today is littered with tier levels divided into pay positions for company employees, so is the military. This being the case, and using the Army enlisted ranks as an example, tier 1 could be regarded as pay grades Private E1 through Specialist E4 (i.e., PFC/E3 Kelly), and tier 2 would be noncommissioned pay grades Corporal E4 through Master Sergeant E8, with tier 3 hitting noncommissioned pay grades First Sergeant E8 and above. I don’t have a scale for that, but that’s how I would envision it.

“Tier level” is a military power-term, and the reference would depend upon the topic under discussion,
Dan