Green’s Dictionary of Slang

1. (US) one’s ‘street’ or gang name; thus a prison nickname.

1985		‘Jennifer Blowdryer’ Modern English 66: placa (n): [...] Your placa is your name, your handle, it can be either sprayed on a wall or in the form of a tattoo.
1992		Bentley & Corbett Prison Sl. 53: Placa Placa means plate or shingle (of a professional person). In prison, placa is the tag or nickname you go by .
1993		L. Rodríguez Always Running (1996) 56: We spray-painted our placas on the wall.
1998		Mayor’s Anti-Gang Office City of Houston, TX Street Gang Sl. / Gloss. 9 Nov.: placas Spanish reference for graffiti identifying a gang name.

2. (US prison) the police; prison officers.

1985		‘Jennifer Blowdryer’ Modern English 66: placa (n): [...] the police, literally: ‘badge’.
1992		Bentley & Corbett Prison Sl. 53: Placa [...] [c]an also be used as a reference to the police.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Placa: Corrrectional Officers. (Sp.).