What is a "Per Diem" worker and how long can they work in true vacancies of civil servant workers?

the public school districts are replacing civil servant (laid-off) "employees from the re-employment list" with a category of workers they are calling Per Diem. Their is no supervision or responsibility from the Civil Service Commission nor seemingly any "legislative language" to define this title. A Per Diem is suppose to be a substitute but the districts are putting them in "true vacancies" under paying former employees (such as myself) and working them "indefinitely". They are denying them the right to join and or form unions and to have union representation, (according to the unions; may be another lawsuit here too). They are paid way less than if they were "called back" from the list and they are deprived of all benefits. The district knows it must fill "true vacancies" with persons from the re-employment list, but they are not acknowledging that these vacancies are true. No one knows how long a Per Diem can fill in because there is no defining the term...