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Dear all,

By using ISYM tag it is possible to switch on/off symmetry for relaxations. but I wonder if it is possible to constrain some of the symmetries but relaxing the others. (Such as imposing a symmetry by giving space group which is different than the group detected by VASP automatically or in an other way?)
Thank you in advance...

in principle, you can only turn off symmetry completely by setting ISYM=0.
Then, the forces are not symmetrized and relaxations are not symmetry-restricted at all.
If the space group you want to impose is lower than the space group detected by vasp, you can in principle try one of the following work-arounds:
1) you can always define equivalent atoms of the high-symmetry cell as inequivalent by spitting their number in POSCAR and simply doubling the PP of the respective atom type in POTCAR. ( like splitting 3 atoms A to 2 1 and copying the PP of A twice to POTCAR)
2) or (if the reduction of the symmetry has the effect that certain atoms only relax along certain directions in space), please choose ISYM=2 in INCAR, but 'selective dynamics' in POSCAR and set the relaxation flags (T/F) for each atom appropriately after the atomic position : i.e. if due to the different space group one atoms relaxes along z only instead of relaxation in xyz , simply set x y z F F T instead of x y z T T T