chr_alt和chr_random和chrUn

chr_alt:

Several human chromosomal regions exhibit sufficient variability to prevent adequate representation by a single sequence. To address this, the GRCh38 assembly provides alternate sequence for selected variant regions through the inclusion of alternate loci scaffolds (or alt loci). Alt loci are separate accessioned sequences that are aligned to reference chromosomes. This assembly contains 261 alt loci, many of which are associated with the LRC/KIR area of chr19 and the MHC region on chr6. See the sequences page for a complete list of the reference chromosomes and alternate sequences in GRCh38.

Haplotype chromosome, unplaced contig and unlocalized contig names now include their NCBI accession number (e.g., chr6_GL000256v2_alt)

The "v2" at the end of the accession number indicates the NCBI version number

Haplotype chromosome names consist of the chromosome number, followed by the NCBI accession number, followed by "alt"

chr_random:

Unlocalized contig names consist of the chromosome number, followed by the NCBI accession number, followed by "random"

chrUn:

Unplaced contig names (contigs whose associated chromosome is not known) consist of "chrUn" followed by the NCBI accession number

ref:http://hgdownload.soe.ucsc.edu/gbdb/hg38/html/description.html