Historically, 400's were CCSO reserves, and 300's were full time deputies. Since they have grown over 100 full time deputies, I believe they have spilled into the 400 series. Here's a breakdown;
100s NBFD
200s NBPD
300s CCSO
400s CCSO
500s Canyone Lake FD/EMS
600s Spring Branch FD
700s Bulverde Fire / 750s + Bulverde PD
800s Bracken Fire
900s Misc county(animal control, constables, judges, garden ridge pd)
1700s Bulverde Spring Branch EMS..
3400s DPS 3B08 troopers
Hope this helps.
Lee