What is the difference between a Type A and Type B Diagnostic Trouble Code?

Type 'A' codes are 'One-Trip' codes, meaning the MIL is commanded on when the failure is first found and a DTC is immediately saved in the computer's memory with Freeze Frame data. Type 'B' DTCs are usually Two-Trip codes, meaning on the first trip the computer sets a Pending DTC but the MIL is not commanded on, and only if the failure is found again on a second consecutive trip does the computer command the MIL on and save the code in long-term memory.