diff --git a/bp2build/testing.go b/bp2build/testing.go
index fd99ff0..f3263a1 100644
--- a/bp2build/testing.go
+++ b/bp2build/testing.go
@@ -76,10 +76,19 @@
 	Description                string
 	ModuleTypeUnderTest        string
 	ModuleTypeUnderTestFactory android.ModuleFactory
-	Blueprint                  string
-	ExpectedBazelTargets       []string
-	Filesystem                 map[string]string
-	Dir                        string
+	// Text to add to the toplevel, root Android.bp file. If Dir is not set, all
+	// ExpectedBazelTargets are assumed to be generated by this file.
+	Blueprint            string
+	// ExpectedBazelTargets compares the BazelTargets generated in `Dir` (if not empty).
+	// Otherwise, it checks the BazelTargets generated by `Blueprint` in the root directory.
+	ExpectedBazelTargets []string
+	Filesystem           map[string]string
+	// Dir sets the directory which will be compared against the targets in ExpectedBazelTargets.
+	// This should used in conjunction with the Filesystem property to check for targets
+	// generated from a directory that is not the root.
+	// If not set, all ExpectedBazelTargets are assumed to be generated by the text in the
+	// Blueprint property.
+	Dir string
 	// An error with a string contained within the string of the expected error
 	ExpectedErr         error
 	UnconvertedDepsMode unconvertedDepsMode