I did. It's not. No help in the Adagio Receivables PDF manual either.

I learned more by finding and examining a sample ARINVIM.XLS file found in the SAMDATA folder. Apparently the flat file Adagio wants is not so flat. One must make an invoice with one row for the header (marked by an "I" in the first column) and then one or more detail lines following (marked by a "D" in the first column). Then the next invoice gets a header row and so forth.

My mistake as assuming that the Adagio Receivables invoice batch import file should look something like the Adagio Receivables invoice batch export file. It does not. This strategy of making import files look like export files had always worked for me in the past.