The UCSC Genome Browser tracks are described in the Table Browser section. Specifically, snp142Common:
This track contains information about a subset of the single
nucleotide polymorphisms and small insertions and deletions (indels) —
collectively Simple Nucleotide Polymorphisms — from dbSNP build 142,
available from ftp.ncbi.nih.gov/snp Only SNPs that have a minor
allele frequency of at least 1% and are mapped to a single location in
the reference genome assembly are included in this subset. Frequency
data are not available for all SNPs, so this subset is incomplete.
The selection of SNPs with a minor allele frequency of 1% or greater
is an attempt to identify variants that appear to be reasonably common
in the general population.
Unlike MGP, which is a very defined sequencing project, dbSNP is a database that accepts submissions from anyone. Thus, there is no guarantee that it is limited to specific strains.