There is NO Section 15 as such in the AIR.
Section 15 describes for the 2019 submission an additional column on Table 6 (on Table 4 for AIR ETC) which the MS can find under section 3.4.
The column “Total eligible expenditure incurred by beneficiaries and paid by 31/12/2018 and certified to the Commission” is only displayed in AIR2018. Its value comes from the “2018 CUM TOTAL” value of the Financial Indicators for each Priority Axis, Fund and Category of Region in Table 5 (in Table 3 for AIR ETC).
The module is based on the model for the annual implementation reports as adopted in the EU Implementing Regulation 2015/2017 (annex V):
https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32015R0207&from=EN