diff --git a/proto/substrait/algebra.proto b/proto/substrait/algebra.proto
index 75a35edc8..924e8dcb6 100644
--- a/proto/substrait/algebra.proto
+++ b/proto/substrait/algebra.proto
@@ -255,6 +255,13 @@ message JoinRel {
   Rel left = 2;
   Rel right = 3;
   Expression expression = 4;
+  // The post-join filter is a filter that is applied to the result of the join before an output
+  // record is produced.  If the filter evaluates to false then the record is not considered a
+  // match.
+  //
+  // For purely logical plans, this field is redundant, as the post_join_filter could be included in
+  // `expression`.  However, for backwards compatibility, and potentially to support engines that do
+  // not support highly logical plans, this field is provided.
   Expression post_join_filter = 5;
 
   JoinType type = 6;
@@ -815,6 +822,9 @@ message HashJoinRel {
   // hash function for a given comparsion function or to reject the plan if it cannot
   // do so.
   repeated ComparisonJoinKey keys = 8;
+  // A boolean condition to be applied to each potential match between the left and right
+  // inputs. If it evaluates to false then the potential match is not considered a match.
+  // This allows the HashJoinRel to express more complex join conditions than just equality.
   Expression post_join_filter = 6;
 
   JoinType type = 7;
@@ -865,6 +875,9 @@ message MergeJoinRel {
   // is free to do so as well).  If possible, the consumer should verify the sort
   // order and reject invalid plans.
   repeated ComparisonJoinKey keys = 8;
+  // A boolean condition to be applied to each potential match between the left and right
+  // inputs. If it evaluates to false then the potential match is not considered a match.
+  // This allows the MergeJoinRel to express more complex join conditions than just equality.
   Expression post_join_filter = 6;
 
   JoinType type = 7;
diff --git a/site/docs/relations/logical_relations.md b/site/docs/relations/logical_relations.md
index 525a65884..adba6b774 100644
--- a/site/docs/relations/logical_relations.md
+++ b/site/docs/relations/logical_relations.md
@@ -234,7 +234,9 @@ The join operation will combine two separate inputs into a single output, based
 | Left Input       | A relational input.                                          | Required                           |
 | Right Input      | A relational input.                                          | Required                           |
 | Join Expression  | A boolean condition that describes whether each record from the left set "match" the record from the right set. Field references correspond to the direct output order of the data. | Required. Can be the literal True. |
-| Post-Join Filter | A boolean condition to be applied to each result record after the inputs have been joined, yielding only the records that satisfied the condition. | Optional                           |
+| Post-Join Filter | A boolean condition to be applied to each potential match between the left and right
+inputs.  If it evaluates to false then the potential match is not considered a match.  A join relation with
+Join Expression X and Post-Join Filter Y is equivalent to a join relation with Join Expression X AND Y. | Optional                           |
 | Join Type        | One of the join types defined below.                         | Required                           |
 
 ### Join Types