Blocks consist of 66 bits. The first two bits of a block are the synchronization header (sync header). Blocks are either data blocks or control blocks. The sync header is 01 for data blocks and 10 for control blocks. Thus, there is always a transition between the first two bits of a block. The remainder of the block contains the payload. The payload is scrambled and the sync header bypasses the scrambler. Therefore, the sync header is the only position in the block that is always guaranteed to contain a transition. This feature of the code is used to obtain block synchronization.
The order of bits of tx_otn_data<N> matches the transmission order described in IEEE 802.3-2015 Clause 82.2.3.2. Specifically, tx_otn_data_<N>[65] is the first sync header bit transmitted. The ordering of blocks is tx_otn_data_0 is the first block transmitted, followed by tx_otn_data_1 up to tx_otn_data_4.
For more information, see IEEE 802.3-2015 Clause 82.2 [Ref 2].