Prison file containing all information on a prisoner or also can mean their reputation. Prisoners can put false jackets on other prisoners to discredit them.
"I'm tired of the motherfuckin' jacket. Sweatin my gang while I'm chillin' in the shack and shining the light in my face and for what? Maybe it's because I kick so much butt." -An exerpt from Eazy's verse on "Fuck tha Police"