diff --git a/proto/substrait/algebra.proto b/proto/substrait/algebra.proto
index a251c945b..60c1d2090 100644
--- a/proto/substrait/algebra.proto
+++ b/proto/substrait/algebra.proto
@@ -205,10 +205,12 @@ message AggregateRel {
   // Input of the aggregation
   Rel input = 2;
 
-  // A list of expression grouping that the aggregation measured should be calculated for.
+  // A list of one or more grouping expression sets that the aggregation measures should be calculated for.
+  // Required if there are no measures.
   repeated Grouping groupings = 3;
 
   // A list of one or more aggregate expressions along with an optional filter.
+  // Required if there are no groupings.
   repeated Measure measures = 4;
 
   substrait.extensions.AdvancedExtension advanced_extension = 10;