This part is associated with phiC31 integration devices
phiC31 is a bacteriophage integrase. It is somewhat similar in mechanism to Cre recombinase, but it acts on the asymmetric sites attB and attP. It causes these sequences to be recombined to generate attL and attR sequences. This system is useful for integrating large DNAs site-specifically into genomes.
You should read the following papers
References: PMID 14996222, PMID 19002165, PMID 19439387, PMID 12034816