purpose of line ".. contents:: :depth: 2" in test-suite.log?

Does anyone know what this line in automake-generated test-suite.log
files is used by or intended for?

. contents:: :depth: 2

It appears to be machine-parsable, but I can't find any reference to it
in the code or the doc. There doesn't seem to be anything that outputs
:depth: 3 (or anything else), either. The above is just a static string
in lib/am/check.am:

	  echo ".. contents:: :depth: 2";				\

Just wondering. I don't plan to change it. --thanks, karl.