[Mp4-tech] Questions about constraint flags

[EricChuang via.com.tw]

Dear experts,
I have a question about the constraint_set0_flag, constraint_set1_flag, ... constraint_set3_flag. I have no idea why this kind of flag is used for. The same restriction can be achieved by changing the profile_idc. 
It seems that these flags can be used to indicate it conforms to baseline, main, or exteded profile, etc.. even if the profile_idc indicates it's high profile. Why not just changing the profile_idc, and created these 4 constraint flags ?
Thanks,
Eric
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