In UCSC genome browser web server there are some chromosome which start with "chrUn", for example chrUn_KI270438v1. What are these chromosome exactly? It means Unknown chromosome? If yes, why unknown?
Quoting directly from the UCSC website:

*"ChrUn contains clone contigs that cannot be confidently placed on a specific chromosome. For the chrN_random and chrUn_random files, we essentially just concatenate together all the contigs into short pseudo-chromosomes. The coordinates of these are fairly arbitrary, although the relative positions of the coordinates are good within a contig."*

https://genome.ucsc.edu/FAQ/FAQdownloads.html