Signals are used in addition to 10 codes. 10 codes usually signify what someone is doing, for example - 10-97 - here, or 10-51 - on the way, while signals are for events/things that happened - by me, signal 4 is accident, signal 7 is body found, signal 5 is murder, etc.
Then they have signal + person or vehicle - so signal 13 person means a suspicious person, while a signal 13 vehicle is a suspicious vehicle.
Here are some from Broward County, FLorida
wiki.radioreference.com
These are the dispatch signals:
0 ARMED & OR CAUTION 46 BOMB THREAT
1 DRUNK DRIVING 47 VICE OP
2 DRUNK PEDESTRIAN 48 OPEN DOOR
3 HIT & RUN 49 ALARM
4 ACCIDENT 50 LABOR TROUBLE
5 MURDER 51 TRESPASSING
6 ESCAPED PRISONER 52 FORGERY-COUNTERFEIT
7 DOA 53 EMBEZZLEMENT
8 MISSING PERSON 54 HIJACK -AIRCRAFT
9 STOLEN TAG 55 EXPLOSION
10 STOLEN CAR 56 BEVERAGE VIOLATION
11 ABANDONED VEH. 57 NARCOTICS
12 RECKLESS DRIVER 58 UNLAWFULL ASSEMBLY
13 SUSPICIOUS INCIDENT 59 FIRE BOMBING
PERSON 60 SNIPER
VEHICLE 61 GAMBLING
14 INFO 62 RACIAL TROUBLE
15 SPECIAL DETAIL 63 IMPERSONATING AN OFFICEER
16 CHILD ABUSE 64 THEFT-AUTO PARTS
17 CONTACT 65 SHOPLIFTEER
18 FELONY 66 CIVIL MATTER
19 MISDEMEANOR 67 SICK OR INJURED PERSON
20 MENTALLY ILL SUBJECT 68 POLICE SERVICE CALL
21 BURGLARY 69 LOOSE ANIMAL
22 DISTURBANCE 70 ANIMAL BITE/ABUSE
23 PEDESTRIAN 71 SNAKE CALL
24 KIDNAPPING 72 LOST/FOUND PROPERTY
25 FIRE 73 TRAFFIC-ALL TYPES
26 DROWNING 74 RECALL
27 PROWLER/PEEPING. TOM 75 AID TO OTHER AGENCY
28 CIVIL DISORDER
29 RECKLESS OPS-BOAT
30 THEFT
31 ASSAULT
32 SUICIDE
33 SHOOTING
34 STABBING
35 RAPE
36 DISTURBANCE -FIGHT
37 DISTURBANCE -JUVENILE
38 DISTURBANCE -DOMESTIC
39 DISTURBANCE -NEIGHBOR
40 VANDALISM/MALICIOUS MISCHIEF
41 ROBBERY
42 CHILD MOLESTER
43 LEWD BEHAVIOUR
44 MARINE ACCIDENT
45 PLANE CRASH
Edit: its interesting you dont hear it much in Arkansas, as here in FL it is used heavily, and on nearly every dispatch call.