diff --git a/CHANGES.txt b/CHANGES.txt
index 1cdbf799c155..153d38585f08 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 5.0.10
+ * Clarify the unset, true and false behaviours of cassandra.reset_bootstrap_progress in the abort message and docs (CASSANDRA-21595)
  * Bound and chunk the BTI partition index preload (CASSANDRA-21632)
  * Restore volatile on CommitLogSegment.cdcState dropped in merge (CASSANDRA-21665)
  * Fix compressed scan read-ahead buffer Block leak and chunk-cache pollution by scans (CASSANDRA-21671)
diff --git a/doc/modules/cassandra/pages/managing/operating/topo_changes.adoc b/doc/modules/cassandra/pages/managing/operating/topo_changes.adoc
index 2bd85519b54c..6ecf3c5c5017 100644
--- a/doc/modules/cassandra/pages/managing/operating/topo_changes.adoc
+++ b/doc/modules/cassandra/pages/managing/operating/topo_changes.adoc
@@ -50,9 +50,41 @@ unavailable replica, set the JVM flag
 On 2.2+, if the bootstrap process fails, it's possible to resume
 bootstrap from the previous saved state by calling
 `nodetool bootstrap resume`. If for some reason the bootstrap hangs or
-stalls, it may also be resumed by simply restarting the node. In order
-to cleanup bootstrap state and start fresh, you may set the JVM startup
-flag `-Dcassandra.reset_bootstrap_progress=true`.
+stalls, it may also be resumed by simply restarting the node.
+
+==== The `cassandra.reset_bootstrap_progress` JVM property
+
+When a bootstrap finds ranges that a previous, partially successful
+bootstrap already marked as available, the behaviour depends on the
+`cassandra.reset_bootstrap_progress` JVM startup property. Leaving it
+unset is *not* the same as setting it to `true` or to `false`.
+
+[cols=",",options="header",]
+|===
+|Value |Behaviour when previous bootstrap progress is found
+
+|unset (default) | If ranges from a previous bootstrap are found,
+the bootstrap usually aborts with an error, so that no decision
+about the existing data is made on your behalf. If none are found,
+the bootstrap proceeds normally.
+
+|`true` |Discards the recorded progress and streams all ranges again.
+The files already streamed are *not* deleted, so remove them before
+retrying or you may resurrect or duplicate data. See
+https://issues.apache.org/jira/browse/CASSANDRA-17679[CASSANDRA-17679].
+
+|`false` |Keeps the recorded progress and streams only the ranges not
+already marked as available. Potentially dangerous, because the data
+streamed by the previous attempt may be missing or incomplete. Prefer
+`true` after cleaning up the old files. See
+https://issues.apache.org/jira/browse/CASSANDRA-17679[CASSANDRA-17679].
+|===
+
+In some situations, a bootstrap with the property unset doesn't abort.
+Instead, it continues and reuses the ranges that a previous attempt
+already streamed, without logging that it skipped them. To avoid this,
+we recommend setting the property to `true` after removing the files
+from the previous attempt.
 
 On lower versions, when the bootstrap proces fails it is recommended to
 wipe the node (remove all the data), and restart the bootstrap process
diff --git a/src/java/org/apache/cassandra/dht/RangeStreamer.java b/src/java/org/apache/cassandra/dht/RangeStreamer.java
index a95a7c300e3e..501d228f08a8 100644
--- a/src/java/org/apache/cassandra/dht/RangeStreamer.java
+++ b/src/java/org/apache/cassandra/dht/RangeStreamer.java
@@ -735,15 +735,18 @@ public StreamResultFuture fetchAsync()
 
                     if (remaining.size() < available.full.size() + available.trans.size())
                     {
-                        // If the operator hasn't specified what to do when we discover a previous partially successful bootstrap,
-                        // we error out and tell them to manually reconcile it. See CASSANDRA-17679.
+                        // If the operator hasn't specified what to do when we discover a previous partially successful bootstrap
+                        // (some of the ranges we are about to fetch are already marked as available), we error out and spell out
+                        // the true/false choice. See CASSANDRA-17679.
                         if (!RESET_BOOTSTRAP_PROGRESS.isPresent())
                         {
                             List<FetchReplica> skipped = fetchReplicas.stream().filter(isAvailable).collect(Collectors.toList());
-                            String msg = String.format("Discovered existing bootstrap data and %s " +
-                                                       "is not configured; aborting bootstrap. Please clean up local files manually " +
-                                                       "and try again or set cassandra.reset_bootstrap_progress=true to ignore. " +
-                                                       "Found: %s. Fully available: %s. Transiently available: %s",
+                            String msg = String.format("Discovered existing bootstrap data (ranges already marked as available by a previous " +
+                                                       "bootstrap attempt) and %1$s is unset; aborting bootstrap. To start a new bootstrap and " +
+                                                       "re-stream all ranges, delete the data files from the previous attempt and set it to 'true'. " +
+                                                       "To stream only the missing ranges, set it to 'false' (potentially dangerous, as the previous " +
+                                                       "data may be incomplete). Set it on restart with -D%1$s=<true|false>. See CASSANDRA-17679. " +
+                                                       "Found: %2$s. Fully available: %3$s. Transiently available: %4$s",
                                                        RESET_BOOTSTRAP_PROGRESS.getKey(), skipped, available.full, available.trans);
                             logger.error(msg);
                             throw new IllegalStateException(msg);