diff --git a/binutils/testsuite/lib/binutils-common.exp b/binutils/testsuite/lib/binutils-common.exp
index 8c6f7ff..a501cf4 100644
--- a/binutils/testsuite/lib/binutils-common.exp
+++ b/binutils/testsuite/lib/binutils-common.exp
@@ -525,6 +525,11 @@ if ![string length [info proc prune_warnings]] {
 #	options in the "source" directives.  See below.
 #       Multiple instances of this directive tells run_dump_test to run the test
 #       multiple times -- one time with each set of flags provided.
+#       Each instance will run exactly as a file with a single "as" line, it is
+#       not possible to condition any behaviour on which set of "as" flags is
+#       used.  That means that the "source" specific options are appended to
+#       the "as" flags for their corresponding files, and any extra processing
+#       (e.g. with "ld" and "objcopy") is repeated for each test.
 #
 #   ld: FLAGS
 #	Link assembled files using FLAGS, in the order of the "source"