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.