diff --git a/pkg/brpb/brpb.pb.go b/pkg/brpb/brpb.pb.go
index 9ad44a1eb..c7b80dbee 100644
--- a/pkg/brpb/brpb.pb.go
+++ b/pkg/brpb/brpb.pb.go
@@ -3960,9 +3960,11 @@ type DataFileInfo struct {
 	RegionEndKey []byte `protobuf:"bytes,21,opt,name=region_end_key,json=regionEndKey,proto3" json:"region_end_key,omitempty"`
 	// The region epoch that the log file belongs to.
 	// In older versions, this might be empty.
-	// If a region get split or merged during observing,
-	// the file may contain multi epoches.
+	// If a region get split or merged during observing, the file may contain multi epoches.
 	// This may not be complete: file may contain records from other versions.
+	//
+	// For now(v8.5), there will only be exactly one epoch due to the restriction of log backup code.
+	// `region_start_key` and `region_end_key` will match this epoch.
 	RegionEpoch []*metapb.RegionEpoch `protobuf:"bytes,22,rep,name=region_epoch,json=regionEpoch,proto3" json:"region_epoch,omitempty"`
 	// Encryption information of this data file, not set if plaintext.
 	FileEncryptionInfo   *encryptionpb.FileEncryptionInfo `protobuf:"bytes,23,opt,name=file_encryption_info,json=fileEncryptionInfo,proto3" json:"file_encryption_info,omitempty"`
@@ -4565,6 +4567,7 @@ func (m *LogFileSubcompaction) GetRegionMetaHints() []*RegionMetaHint {
 
 type RegionMetaHint struct {
 	// The left boundary of the region of this epoch.
+	// It is an byte-comparable encoded key without TS.
 	StartKey []byte `protobuf:"bytes,1,opt,name=start_key,json=startKey,proto3" json:"start_key,omitempty"`
 	// The right boundary of the region of the epoch.
 	EndKey []byte `protobuf:"bytes,2,opt,name=end_key,json=endKey,proto3" json:"end_key,omitempty"`
diff --git a/proto/brpb.proto b/proto/brpb.proto
index b00c166f0..0ca030e14 100644
--- a/proto/brpb.proto
+++ b/proto/brpb.proto
@@ -739,9 +739,11 @@ message DataFileInfo {
     bytes region_end_key = 21;
     // The region epoch that the log file belongs to.
     // In older versions, this might be empty.
-    // If a region get split or merged during observing,
-    // the file may contain multi epoches.
+    // If a region get split or merged during observing, the file may contain multi epoches.
     // This may not be complete: file may contain records from other versions.
+    //
+    // For now(v8.5), there will only be exactly one epoch due to the restriction of log backup code.
+    // `region_start_key` and `region_end_key` will match this epoch.
     repeated metapb.RegionEpoch region_epoch = 22;
 
     // Encryption information of this data file, not set if plaintext.
@@ -812,6 +814,7 @@ message RegionMetaHint {
     // You may get region id from `LogFileSubcompactionMeta`.
 
     // The left boundary of the region of this epoch.
+    // It is an byte-comparable encoded key without TS.
     bytes start_key = 1;
     // The right boundary of the region of the epoch.
     bytes end_key = 2;