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After each ionic step and at the end of each job a CONTCAR file is written. This file has a valid POSCAR format and can be used for "continuation" jobs. After each ionic step whether the file can be read or not depends on the operating system. VASP writes, flushes and rewinds the file. If you stop or kill VASP it should be possible to continue from the CONTCAR file.
For MD-runs (IBRION=0) the CONTCAR file contains the actual coordinates, velocities and predictor-corrector coordinates needed as an input for the next MD-job.
For relaxation jobs the CONTCAR file contains the positions of the last ionic step of the relaxation. If the relaxation run has not yet converged one should copy CONTCAR to POSCAR before continuing. For static calculations CONTCAR is identical to POSCAR.