Characters are what the user is trying to send. There are data characters, command characters and transmission characters. The reason the term byte is not used is because different interfaces have different character sizes. For example, the CY7C924ADX (or CY7C9689A) can have 8 or 10 bit data characters, and 10 or 12-bit transmission characters (a transmission character is a character after the encoding overhead has been added).
There is a direct relationship between these. On the CY7B923/933, the character rate is always 1/10th the signaling rate; e.g. for a 270 Mbps signaling rate the character rate is 27 MHz. On the 924ADX/9689A this relationship depends on the SPDSEL and RANGESEL inputs (see Table 4 pg. 17 of 924ADX datasheet for example).
HOTLink Transmitter/Receiver (Attached below 38_02017)
TAXI-compatible HOTLink Transceiver ( Attached below 38-02020_0C_V)
200-MBaud HOTLink Transceiver (Attached below 38-02008_0D_V)