The P{RS3r} transgene is generated in vivo by FLP-mediated excision of the FRT cassette in P{RS3}; this produces a "remnant" element that contains a single FRT site upstream of a non-functional 3' portion of w.
An insertion of P{RS3r} can be used in conjunction with an insertion of P{RS5r} to induce a chromosomal rearrangement whose breakpoints are defined by the location of the original insertions.