"us" means microseconds.
The specification of 1500-1900 microseconds for clockwise rotation means that from 1500 us (center) it will rotate clockwise as it goes to 1900 microseconds, and it will be approximately 45 degrees clockwise rotated at 1900 microseconds. This means it should be at 90 degrees clockwise at 2300 microseconds, and 90 degrees counterclockwise at 700 microseconds.
The measurement of the pulse width is the microseconds measurement. 1500 microseconds is 1.5 milliseconds. A 50 Hz control rate sends a pulse every 20 milliseconds (every 20000 microseconds.)