I have learned in my contracting career that BLock 13C is used for bi-lateral modifications and Block 13D is used for uni-lateral modifications as explained below:
13C: use it when a specific authority (FAR, DFARS, etc.) is cited.
13D: use it when no specific authority is cited, but something that is agreed on "by mutual agreement of parties" or a specific authority is cited for uni-lateral modifications (e.g., option exercise, etc.)
Someone commented that 13D should be used ONLY when the mod is "unilateral."
Thank you in advance for your help.
The following is a reasonable interpretation of the intent of blocks 13A through D, in the absence of more specific instructions on the form itself:
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13A: Clearly for changes pursuant to the Changes Clause
13B: Clearly only for administrative changes
13C: Clearly only for supplemental (bilateral) agreements
13D: Since blocks A through C cover the majority of contract modifications, this block is for any modifications that don't fall into blocks A through C. One example is for a unilateral modification that is not actually a Change Order, i.e., is not issued pursuant to a Changes clause. This might be a modification pursuant to an Option clause or Property clause. I see no reason to use 13D for a bilateral modification when 13C already exists for them.