score:6
According to this article, the TSA officer signs your printed boarding pass so that if questioned, TSA knows who checked you through.
...depending on the airport and the management at the airport, there may also be further information, such as the number of the checkpoint.
The article also states that some TSA officers purposely make the marking illegible just in case they let someone through who needed additional screening, management wouldn't be able to trace it back to that officer. The article essentially states that this process is silly because it can't be done with mobile or digital boarding passes.
...Itβs all weird, since they let phone boarding passes through with no marks...