Per 1099-C instructions,
"Box 6. Shows the reason your creditor has filed this form. The codes in this box
are described in more detail in Pub. 4681. A—Bankruptcy; B—Other judicial
debt relief; C—Statute of limitations or expiration of deficiency period; D—
Foreclosure election; E—Debt relief from probate or similar proceeding; F—By
agreement; G—Decision or policy to discontinue collection; or H—Other actual
discharge before identifiable event."
Here's a link to IRS Publication 4681, Cancelled Debts, which discusses all aspects of debt cancellation, including box 6 codes (see page 3): https://www.irs.gov/pub/irs-pdf/p4681.pdf.