how to choose parameter of prec

[weiyidan]

dear professor,
i find the results of normal and accurate are not same. therefore, i am wondering about how to choose this parameter.
1.is the prec the same as the one of Structural optimization？
2.for Clneb, the accurate is better than normal? (the manual of vasp says 'PREC=Accurate increases the memory requirements somewhat, but it
should be used if very accurate forces (phonons and second derivatives) are required')

thanks for attention and reply.