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