The P element has found wide use in Drosophila research as a mutagen. The mutagenesis system typically uses an autonomous but immobile element, and a mobile nonautonomous element. Flies from subsequent generations can then be screened by phenotype or PCR.  Naturally-occurring P elements contain coding sequence for the enzyme transposase and recognition sequences for transposase action.  Transposase regulates and catalyzes the excision of a P element from the host DNA, cutting at the two recognition sites, and then reinserting randomly. It is the random insertion that may interfere with existing genes, or carry an additional gene, that can be used for genetic research.
To use this as a useful and controllable genetic tool, the two parts of the P element must be separated to prevent uncontrolled transposition. The normal genetic tools are DNA coding for transposase with no transposase recognition sequences so it cannot insert and a "P Plasmid".  P Plasmids always contain a Drosophila reporter gene, often a red-eye marker (the product of the white gene), and transposase recognition sequences. They may contain a gene of interest, an E. coli selectable marker gene, often some kind of antibiotic resistance, an origin of replication or other associated plasmid "housekeeping" sequences.