IDV 1 Integer Maximum number of reads supporting an indel
IMF 1 Float Maximum fraction of reads supporting an indel

based on the introduction here: https://support.bioconductor.org/p/78318/

It doesn't appear to be documented.

The relevant section from the samtools code is

for (s = 0; s < n; ++s) {
    int na = 0, nt = 0;
    for (i = 0; i < n_plp[s]; ++i) {
        const bam_pileup1_t *p = plp[s] + i;
        if (rghash == 0 || p->aux == 0) {
            ++nt;
            if (p->indel != 0) {
                ++na;
                aux[m++] = MINUS_CONST + p->indel;
            }
        }
        j = bam_cigar2qlen(p->b->core.n_cigar, bam_get_cigar(p->b));
        if (j > max_rd_len) max_rd_len = j;
    }
    double frac = (double)na/nt;
    if ( !indel_support_ok && na >= bca->min_support && frac >= bca->min_frac )
        indel_support_ok = 1;
    if ( na > bca->max_support && frac > 0 ) bca->max_support = na, bca->max_frac = frac;
    n_alt += na;
    n_tot += nt;
}

IDV is bca->max_support and IMF is bca->max_frac.