SID is code for special internal investigation. The inmate is under investigation for an infraction of sorts. Normally, any inmate under investigation is relegated to the SHU (the hole, isolation) and cannot receive visits until there is a decision handed down.
https://www.inmateaid.com/ask-the-inmate/my-son-was-in-bayside-he-got-transfered-to-northern-state-i-got-letter-saying-no-visits-due-to-sid-investigation-i-dont-know-what-happened-or-what-sid-means-can-you-help#answer
Accepted Answer Date Created: April 27,2019