All packets are sent reliable and ordered.
Delta compression is used to send only the difference between the last sent state and the current state. For example if you synchronize a Transform. Scale x, y, z is most likely going to keep the same value. Unity compares the state received by the client to the new state, if it is changed 1 bit and the data will be sent. If it didn't change only a single bit will be sent. When Unity detects that nothing has changed since the last state, no data will be sent at all.