Both inputs come from the financing section of the cash flow statement. Buybacks are usually tagged PaymentsForRepurchaseOfCommonStock in XBRL, or PaymentsForRepurchaseOfEquity when preferred stock is included. Issuance is typically ProceedsFromIssuanceOfCommonStock, and many companies also report proceeds from stock option exercises and employee purchase plans on separate lines. Under ASC 230 all of these are financing cash flows. Repurchased shares are either retired or held as treasury stock, which is shown as a reduction of equity.
Each input is summed over the four most recent quarters. Since 10-Q cash flow statements are cumulative from the start of the fiscal year, the trailing amount is the last annual figure plus the current year-to-date figure minus the prior year's matching year-to-date figure. Sign conventions vary: in the filing, repurchases are negative and proceeds positive, and data sources may show the net as either a positive outflow or a negative number, so check the convention before comparing.
Gross buybacks can overstate what shareholders receive. A company that issues large amounts of stock to employees may spend heavily on repurchases just to keep its share count flat. Net repurchases capture that offset, though shares issued directly as compensation, with no cash received, do not appear here at all and show up only in the share count. Analysts add TTM net repurchases to TTM dividends paid to measure total cash returned, and divide by market capitalization to get the buyback component of shareholder yield.