"P71" refers to the three characters in the VIN number of a the Ford Crown Victoria sedan. In other words, "P71" defines the car as a "Police Interceptor." The P71 is manufactured with heavy duty parts, dual exhaust, faster engine, etc.
2004 Ford Crown Victoria LX: 2FAFP74WX4X147971
2000 P71: 2FAFP71W4YX148656