diff --git a/tsdb/compact.go b/tsdb/compact.go
index cb5e8d4c16..216036cfca 100644
--- a/tsdb/compact.go
+++ b/tsdb/compact.go
@@ -947,6 +947,8 @@ type DefaultBlockPopulator struct{}
 // PopulateBlock fills the index and chunk writers with new data gathered as the union
 // of the provided blocks. It returns meta information for the new block.
 // It expects sorted blocks input by mint.
+// If there is more than 1 output block, each output block will only contain series that hash into its shard
+// (based on total number of output blocks).
 func (c DefaultBlockPopulator) PopulateBlock(ctx context.Context, metrics *CompactorMetrics, logger log.Logger, chunkPool chunkenc.Pool, mergeFunc storage.VerticalChunkSeriesMergeFunc, concurrencyOpts LeveledCompactorConcurrencyOptions, blocks []BlockReader, minT, maxT int64, outBlocks []shardedBlock) (err error) {
 	if len(blocks) == 0 {
 		return errors.New("cannot populate block(s) from no readers")