An STR multiplex system is used to identify specific short tandem repeats (STRs).
STR polymorphisms are genetic markers that may be used to identify a DNA sequence.
The FBI analyses 13 specific STR loci for their database.
One can think of a STR multiplex system as a collection of specific STRs which are positionally conserved on a target genome.
For example, the STR multiplex system AmpFlSTR Profiler Plus which analyses nine different STRs (3S1358, vWA, FGA, D8S1179, D21S11, D18S51, D5S818, D13S317, D7S820) plus Amelogenin for sex determination is used for human identification purposes.