diff --git a/deepspeech/models/u2.py b/deepspeech/models/u2.py
index 9337a5bbf3e9400199ed3ca648d78689850082a9..8091dbe14994ef491dea662267287fec400f0bac 100644
--- a/deepspeech/models/u2.py
+++ b/deepspeech/models/u2.py
@@ -399,6 +399,7 @@ class U2BaseModel(nn.Module):
         assert speech.shape[0] == speech_lengths.shape[0]
         assert decoding_chunk_size != 0
         batch_size = speech.shape[0]
+
         # Let's assume B = batch_size
         # encoder_out: (B, maxlen, encoder_dim)
         # encoder_mask: (B, 1, Tmax)
@@ -410,10 +411,12 @@ class U2BaseModel(nn.Module):
         # encoder_out_lens = encoder_mask.squeeze(1).sum(1)
         encoder_out_lens = encoder_mask.squeeze(1).astype(paddle.int).sum(1)
         ctc_probs = self.ctc.log_softmax(encoder_out)  # (B, maxlen, vocab_size)
+
         topk_prob, topk_index = ctc_probs.topk(1, axis=2)  # (B, maxlen, 1)
         topk_index = topk_index.view(batch_size, maxlen)  # (B, maxlen)
         pad_mask = make_pad_mask(encoder_out_lens)  # (B, maxlen)
         topk_index = topk_index.masked_fill_(pad_mask, self.eos)  # (B, maxlen)
+
         hyps = [hyp.tolist() for hyp in topk_index]
         hyps = [remove_duplicates_and_blank(hyp) for hyp in hyps]
         return hyps
@@ -449,6 +452,7 @@ class U2BaseModel(nn.Module):
         batch_size = speech.shape[0]
         # For CTC prefix beam search, we only support batch_size=1
         assert batch_size == 1
+
         # Let's assume B = batch_size and N = beam_size
         # 1. Encoder forward and get CTC score
         encoder_out, encoder_mask = self._forward_encoder(
@@ -458,7 +462,9 @@ class U2BaseModel(nn.Module):
         maxlen = encoder_out.shape[1]
         ctc_probs = self.ctc.log_softmax(encoder_out)  # (1, maxlen, vocab_size)
         ctc_probs = ctc_probs.squeeze(0)
+
         # cur_hyps: (prefix, (blank_ending_score, none_blank_ending_score))
+        # blank_ending_score and  none_blank_ending_score in ln domain
         cur_hyps = [(tuple(), (0.0, -float('inf')))]
         # 2. CTC beam search step by step
         for t in range(0, maxlen):
@@ -498,6 +504,7 @@ class U2BaseModel(nn.Module):
                 key=lambda x: log_add(list(x[1])),
                 reverse=True)
             cur_hyps = next_hyps[:beam_size]
+
         hyps = [(y[0], log_add([y[1][0], y[1][1]])) for y in cur_hyps]
         return hyps, encoder_out
 
@@ -561,12 +568,13 @@ class U2BaseModel(nn.Module):
         batch_size = speech.shape[0]
         # For attention rescoring we only support batch_size=1
         assert batch_size == 1
-        # encoder_out: (1, maxlen, encoder_dim), len(hyps) = beam_size
+
+        # len(hyps) = beam_size, encoder_out: (1, maxlen, encoder_dim)
         hyps, encoder_out = self._ctc_prefix_beam_search(
             speech, speech_lengths, beam_size, decoding_chunk_size,
             num_decoding_left_chunks, simulate_streaming)
-
         assert len(hyps) == beam_size
+
         hyps_pad = pad_sequence([
             paddle.to_tensor(hyp[0], place=device, dtype=paddle.long)
             for hyp in hyps
@@ -576,23 +584,29 @@ class U2BaseModel(nn.Module):
             dtype=paddle.long)  # (beam_size,)
         hyps_pad, _ = add_sos_eos(hyps_pad, self.sos, self.eos, self.ignore_id)
         hyps_lens = hyps_lens + 1  # Add <sos> at begining
+
         encoder_out = encoder_out.repeat(beam_size, 1, 1)
         encoder_mask = paddle.ones(
             (beam_size, 1, encoder_out.shape[1]), dtype=paddle.bool)
+
         decoder_out, _ = self.decoder(
             encoder_out, encoder_mask, hyps_pad,
             hyps_lens)  # (beam_size, max_hyps_len, vocab_size)
+        # ctc score in ln domain
         decoder_out = paddle.nn.functional.log_softmax(decoder_out, axis=-1)
         decoder_out = decoder_out.numpy()
+
         # Only use decoder score for rescoring
         best_score = -float('inf')
         best_index = 0
+        # hyps is List[(Text=List[int], Score=float)], len(hyps)=beam_size
         for i, hyp in enumerate(hyps):
             score = 0.0
             for j, w in enumerate(hyp[0]):
                 score += decoder_out[i][j][w]
+            # last decoder output token is `eos`, for laste decoder input token.
             score += decoder_out[i][len(hyp[0])][self.eos]
-            # add ctc score
+            # add ctc score (which in ln domain)
             score += hyp[1] * ctc_weight
             if score > best_score:
                 best_score = score
diff --git a/deepspeech/modules/encoder.py b/deepspeech/modules/encoder.py
index 0aedea7480a72f627e3f313b04ecfe0add7ad4de..af782fb53fa5459e93bb6e99b2a4202bdb4121a4 100644
--- a/deepspeech/modules/encoder.py
+++ b/deepspeech/modules/encoder.py
@@ -219,11 +219,14 @@ class BaseEncoder(nn.Layer):
 
         xs, pos_emb, _ = self.embed(
             xs, tmp_masks, offset=offset)  #xs=(B, T, D), pos_emb=(B=1, T, D)
+
         if subsampling_cache is not None:
             cache_size = subsampling_cache.size(1)  #T
             xs = paddle.cat((subsampling_cache, xs), dim=1)
         else:
             cache_size = 0
+
+        # only used when using `RelPositionMultiHeadedAttention`
         pos_emb = self.embed.position_encoding(
             offset=offset - cache_size, size=xs.size(1))
 
@@ -237,7 +240,7 @@ class BaseEncoder(nn.Layer):
 
         # Real mask for transformer/conformer layers
         masks = paddle.ones([1, xs.size(1)], dtype=paddle.bool)
-        masks = masks.unsqueeze(1)  #[B=1, C=1, T]
+        masks = masks.unsqueeze(1)  #[B=1, L'=1, T]
         r_elayers_output_cache = []
         r_conformer_cnn_cache = []
         for i, layer in enumerate(self.encoders):