R. Stanton Scott revisits Thomas Adams article in Parameters (Summer 1999) with his categorization of PMCs, but there is more to the taxonomy than services offered. In his foundational article, Adams describes three types of mercenaries.
Heavily weighted, the term of "mercenary" has come to be associated with Executive Outcomes and Sandline. Mercenaries are typically cast as individualistic, Rambo soldiers of fortune. With large corporations such as NorthrupGrumman (Vinnell) and CSC (DynCorp) behind some of the more prolific private military companies, another, more fluid taxonomy was required than Adams' three baskets.
P.W. Singer, in Corporate Warriors, uses a spear analogy to describe how far or near the firms are from "implementation" in combat. Singer maintains the three general types of Adams with his own terms: Military Provider Firms, Military Consultant Firms, and Military Support Firms. His "tip of the spear" typology allows for granular shifting along an axis towards or away service offerings.
The singular x-axis plotting is inadequate, however. It ignore a substantial descriptor of the firms that I believe is crucial when understanding their participation in the state vs non-state structure: location of headquarters.
The institutionalized system of state and non-state relationships is interconnected with limitations on the civilian leadership of the private military company. The location of the HQ grants or prohibits legal action by legalist states, thereby promoting various actions by the principals of the PMC.
For example, an operation such as Tim Spicer's Sandline (now defunct), based off-shore from the United Kingdom, provided relief from potential legal actions. When investigating Sandline in the Sri Leone affair, found that even if they could take action against his corporation, they were limited because of its location. (See Private Military Companies: Options for Regulations). This holds true when attempting to put pressure on the HQ host government (see Annex B of previous).
On the y-axis would be three marks, just like the x-axis. These three would indicate the nature of the state and indicate the personality of the firm. The first mark would be a Western industrialized state such as the United States, Britain, Germany, etc. These states have deeply ingrained civil-military relationships (or civil-society-military, but that's a different discussion) and institutionalized legal and financial systems (sticks and carrots).
The second bucket would be weak states without the distinct civil-military relations and more permiable institutions. These would include Belarus, Israel, or others. Israel is an interesting inclusion mostly because of its military with too much control of the political process and willingness to provide services to those who will pay (also known as the "biting the hand that feds syndrome").
The last mark would be HQ's in the outlaw states such as Afghanistan or North Korea. This is not to say NK has a PMC, but such a "corporate" entity would be by definition not independent and would be the tool of the state. These would either be better classified as pirates or state
This two dimensional plotting lends itself to the accountability question, to be discussed soon.