this command is used for explicitly defining test clock in the design.
here you are defining ScanClock Timing as {45 55}.This waveform definition consists of two values: first rising-edge arrival time and then falling-edge arrival time. this is for multiplexed flip-flop style.
here you have 100ns clock period (which is by default), for your return-to-zero clock during that your positive edge of the clock will arrive at 45ns and your negative edge of the clock will arrive at 55ns.
scan clock is always slower than the functional clock, for more understanding please refer fundamentals of clock period, clock duty cycle and clock frequency and you will understand.
if you don't want to define explicitly then use -infer_clock with -create_test_protocol and tool will take default timing for whichever flop style you are using.