I have downloaded the human genomic DNA from the UCSC genomic browser website from the link, hg38.chromFa.tar.gz . When I extracted, I got all the chromosomal sequences. While I got the chromosome 1 as chr1.fa, I also see several shorter chr1 sequences with names such as chr1_GL383518v1_alt.fa, chr1_KI270706v1_random.fa, chr1_KI270759v1_alt.fa to name a few. There are a total of twenty one such short sequences for chr1. I couldn't find any documentation in the website about these sequences. I would like to know what are these sequences.