Available up from VASP version 3.2.
The ELFCAR file is created when the LELF flag (see section 6.55)
in the INCAR file is set to .TRUE.
and contains the so-called ELF (electron localization function).
It has the same format as the CHG file. It is recommended to avoid wrap around errors, when evaluating ELFCAR file. This can be done by specifying PREC=High in the INCAR file.