A TTL signal, Transistor-Transistor-Logic, is a binary signal generated with the help of a transistor turning on or off. 0-0.4V is considered as low state, and 2.7-5V is a high state.
The difference between the TTL and Relay Contact topologies is that since in Mean Well products the TTL signal is created through a transistor or semiconductor, the current capability of the signal is relatively low. In contrary, using Relay Contact method (which uses a metal contact as conductor) allows much higher current to flow.