FAR 2.101 defines "Contracting officer" (CO) as "a person with the authority to enter into, administer, and/or terminate contracts and make related determinations and findings. The term includes certain authorized representatives of the contracting officer acting within the limits of their authority as delegated by the contracting officer. "Administrative contracting officer (ACO)" refers to a contracting officer who is administering contracts. "Termination contracting officer (TCO)" refers to a contracting officer who is settling terminated contracts. A single contracting officer may be responsible for duties in any or all of these areas. Reference in this regulation (48 CFR chapter 1) to administrative contracting officer or termination contracting officer does not-

           (1) Require that a duty be performed at a particular office or activity; or

           (2) Restrict in any way a contracting officer in the performance of any duty properly assigned."

Additionally, FAR 2.101 defines "Contract" as a mutually binding legal relationship obligating the seller to furnish the supplies or services (including construction) and the buyer to pay for them. It includes all types of commitments that obligate the Government to an expenditure of appropriated funds and that, except as otherwise authorized, are in writing. In addition to bilateral instruments, contracts include (but are not limited to) awards and notices of awards; job orders or task letters issued under basic ordering agreements; letter contracts; orders, such as purchase orders, under which the contract becomes effective by written acceptance or performance; and bilateral contract modifications. Contracts do not include grants and cooperative agreements covered by 31 U.S.C.6301, et seq. For discussion of various types of contracts, see part 16"

Also, the FAR defines "contract action" in a few different places in the FAR and DFARS including FAR Parts 4, 5, 32 and DFARS Part 217.  However, the definition in FAR 4.601 helps us the best to understand the meaning of "contract action" as it's used in AFFARS MP5301.603-90(i).  FAR 4.601 defines "Contract action" as "any oral or written action that results in the purchase, rent, or lease of supplies or equipment, services, or construction using appropriated dollars over the micro-purchase threshold, or modifications to these actions regardless of dollar value. Contract action does not include grants, cooperative agreements, other transactions, real property leases, requisitions from Federal stock, training authorizations, or other non-FAR based transactions."

Based on this definition of "contract action" from FAR 4.601 and my reading of AFFARS MP5301.603-90(i), it seems pretty clear to me that each contract modification would be considered a separate "contract action".  And the example in AFFARS MP5301.603-90(i) demonstrates how to apply the dollar limit of the contracting officer's warrant to the value of the "contract action" as opposed to the overall value of the original contract.  The example reads, "if a CO holds a limited warrant of $5M and the contract award amount of the instant action is $6M, the CO is not authorized to sign the contract; however, the same CO would be authorized to sign a $5M funding action issued against the same contract."  It might help if we look at this same example in a little more detail.  Let's assume when the original contract for $6M was awarded, it was signed by a CO with a $6M warrant.  In this case, the $6M contract award is considered the "instant action", and the CO, in this instance, had the authority to sign it (because she had a $6M warrant).  Now a contract modification to add $5M in funding to the original $6M contract needs to be signed, but the CO with a $6M warrant has moved on and the new CO only has a $5M warrant.  In this case, the $5M funding action would be considered the "instant contract action", and the new CO (who has a $5M warrant) is authorized to sign the $5M modification.  Let's look at another example using the same contract.  Let's assume the first modification to add $5M to the contract has been awarded, and the new total value of the contract is $11M.  Now a second funding action to add another $2M to the contract is required.  Once again, our CO with a $5M warrant has the authority to sign this modification because it's based on the $2M value of the modification and not the total contract value of $11M. 

You also asked about how to apply the dollar limit in the CO's warrant to a contract termination.  FAR 49.101(a) states "the termination clauses or other contract clauses authorize contracting officers to terminate contracts for convenience, or for default, and to enter into settlement agreements under this regulation."  Also, FAR 2.1.1 defines "terminated portion" of the contract as "the portion of a contract that the contractor is not to perform following a partial termination. For construction contracts that have been completely terminated for convenience, it means the entire contract, notwithstanding the completion of, and payment for, individual items of work before termination."  According to FAR 2.101 a "contracting officer" or CO has "the authority to enter into, administer, and/or terminate contracts...."  The CO's warrant should specifically say if this person has the authority to terminate contracts and the dollar limit.  The value of the termination action (the "instant contract action" in this case) cannot exceed the dollar limit specified in the warrant.  For example, if the total contract is being terminated, then the dollar limit would apply to the estimated or actual amount of the termination settlement for the terminated contract.  If it's a partial termination, it would apply to the estimated value of the terminated portion as defined in FAR 2.101. 

Finally, you asked about the meaning of "per transaction" in AFFARS 5301.603-3(b)(1).  However, this reference has to do with appointing decentralized ordering officers from other organizations within a government agency.  Their appointments authorize them to place orders directly off of a decentralized contract or agreement.  It also refers to GPC cardholders.  It is NOT referring to the appointment of Contracting Officers.

Bottom line: the dollar limit of the CO warrant is applied against the value of the "instant contract action" and not the overall contract value.  At least, this was how I read AFFARS MP5301.603-90(i), but I could see how you might have a different interpretation.   The folks in SAF/AQC wrote the policy and would be able to provide a more definitive answer to your question.  I recommend contacting the SAF/AQC Action Officer for FAR Part 01, Ms. Karen Stevens, at [email protected] or 571-256-2394.  I, also, recommend going up your own contracting chain of command to check your local agency's CO warrant policy, which could be different from SAF/AQC's.