Sentence:  In RTU (Remote Terminal Unit) mode, each 8-bit byte in a message contains two 4-bit hexadecimal characters. The main advantage of this mode is that its greater <WSD>character density</WSD> allows better data throughput than ASCII for the same baud rate. 
Meaning of the word marked with <WSD> tags:  A measure of the number of characters that are represented in a specified unit of text stream.