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I've never heard of rententivity, it sounds the same as remanence though.
'Square' ferrite has high remenance.
Coercivity is the magnetic flux applied to magnetize the ferrite from a winding, it depends on the application as to whether you want the ferrite to be high or low, for a flyback smps you'd want ferritte that can withstand high coercivity, a common unit for coercivity is oersteds.