What does code j mean on police scanner?

Law enforcement codes and signals are not nationally standardized. Each department has a set that is specific to their department. Most departments don't even use letter codes, they would be numbers.

In many departments, a J might be added to the end of another signal to signify that something had just occurred. It would be a modifier, not the entire code.

For instance, in Tampa, a signal 10 is a stolen vehicle. I signal 10J would be a stolen vehicle that just occurred.