These flags should be set according to the computer on which VASP runs.
In some cases VASP has to perform an unitary transformation of the current wave functions. This is done using a work array CBLOCK and the following FORTRAN code:
DO 100 IBLOCK=0,NPL-1,NBLK
ILEN=MIN(NBLK,NPL-IBLOCK)
DO 200 N1=1,N
DO 200 M=1,ILEN
CBLOCK(M,N1)=C(M+IBLOCK,N1)
C(M+IBLOCK,N1)=0
200 CONTINUE
C C(IBLOCK+I,N)=SUM_(J,K) CH(I,K) CBLOCK(K,N)
CALL ZGEMM ('N', 'N', ILEN, N, N, (1.,0.), CBLOCK, NBLK, CH, N,
& (1.,0.), C(IBLOCK+1,1), NDIM)
100 CONTINUE
ZGEMM is the matrix 
matrix multiplication command of the
BLAS package. The task performed by this call is indicated by the
comment line written above the ZGEMM call.
Generally NBLK=16 or 32 is large enough for super-scalar machines.
A large value might be necessary on vector machines for optimal
performance (NBLK=128).
(In VASP.4.X. NBLK is can be read from the INCAR file during run time).
MCPU is not supported.