about ADS-B’ usage
 

in icao doc 4444，the section 8.1.11 says：
ADS-B may be used alone, including in the provision of separation between aircraft, provided:
a) identification of ADS-B-equipped aircraft is established and maintained;
b) the data integrity measure in the ADS-B message is adequate to support the separation minimum;
c) there is no requirement for detection of aircraft not transmitting ADS-B; and
d) there is no requirement for determination of aircraft position independent of the position-determining elements
of the aircraft navigation system.
and 8.1.9 says：
SSR systems, especially those utilizing monopulse techniques or having Mode S capability or MLAT, may be used alone, including in the provision of separation between aircraft, provided:
a) the carriage of SSR transponders is mandatory within the area; and
b) identification is established and maintained.
how to interpret the item c and d of 8.1.11？Why are the differences between the requirements for secondary radar and ADS-B are so big

Hopefully someone with a more detailed technical knowledge than me will provide a better answer but, in essence, SSR 'only' relies of the transponder's response to the interrogation signal and the position of the aircraft is derived from calculations made by the surveillance system processors. With ADS-B, the system is reliant upon the GNSS equipment on board the aircraft for the positional data and the 'quality' of that can vary significantly across the various equipment that is available and can be subject to jamming of the GNSS signal.