Legal Question in Criminal Law in California

Generally, it means this: a person is on probation on a case (Case A) and has another case (Case B) and is then sentenced to prison on Case B; The person, or their probation officer or the warden, notifies the judge in Case A that the person is in prison on Case B; the judge in Case A can sentence the person to prison for Case A and the term of imprisonment is counted from the date the person was committed to prison on Case B. There are other specifics about that section, but this is essentially it.