diff --git
a/streams/src/main/java/org/apache/kafka/streams/processor/internals/PartitionGroup.java
b/streams/src/main/java/org/apache/kafka/streams/processor/internals/PartitionGroup.java
index 1fdd454ea54..fbafa73aca2 100644
---
a/streams/src/main/java/org/apache/kafka/streams/processor/internals/PartitionGroup.java
+++
b/streams/src/main/java/org/apache/kafka/streams/processor/internals/PartitionGroup.java
@@ -27,14 +27,26 @@
import java.util.Set;
/**
- * A PartitionGroup is composed from a set of partitions. It also maintains
the timestamp of this
- * group, a.k.a. the stream time of the associated task. It is defined as the
maximum timestamp of
- * all the records having been retrieved for processing from this
PartitionGroup so far.
+ * PartitionGroup is used to buffer all co-partitioned records for processing.
*
- * We decide from which partition to retrieve the next record to process based
on partitions' timestamps.
- * The timestamp of a specific partition is initialized as UNKNOWN (-1), and
is updated with the head record's timestamp
- * if it is smaller (i.e. it should be monotonically increasing); when the
partition's buffer becomes empty and there is
- * no head record, the partition's timestamp will not be updated any more.
+ * In other words, it represents the "same" partition over multiple
co-partitioned topics, and it is used
+ * to buffer records from that partition in each of the contained
topic-partitions.
+ * Each StreamTask has exactly one PartitionGroup.
+ *
+ * PartitionGroup implements the algorithm that determines in what order
buffered records are selected for processing.
+ *
+ * Specifically, when polled, it returns the record from the topic-partition
with the lowest stream-time.
+ * Stream-time for a topic-partition is defined as the highest timestamp
+ * yet observed at the head of that topic-partition.
+ *
+ * PartitionGroup also maintains a stream-time for the group as a whole.
+ * This is defined as the highest timestamp of any record yet polled from the
PartitionGroup.
+ * The PartitionGroup's stream-time is also the stream-time of its task and is
used as the
+ * stream-time for any computations that require it.
+ *
+ * The PartitionGroups's stream-time is initially UNKNOWN (-1), and it set to
a known value upon first poll.
+ * As a consequence of the definition, the PartitionGroup's stream-time is
non-decreasing
+ * (i.e., it increases or stays the same over time).
*/
public class PartitionGroup {
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