Branding
The first use of the PR system was in 1996, when AMD used it to assert that their AMD 5x86 processor was as fast as a Pentium running at 75 MHz. The designation "P75" was added to the chip to denote this.
The letters PR stood for "Performance Rating", but many people mistakenly thought it stood for "Pentium Rating", as the PR was often used to measure performance against Intel's Pentium processor.
Later that year, Cyrix also adopted the PR system for its 6x86 and 6x86MX line of processors. These processors were capable of handling business applications under Microsoft Windows faster than Pentiums of the same clock speed, so Cyrix PR-rated the chips one or two Pentium speed grades higher than clock speed. AMD did likewise with some versions of their K5 processor, but abandoned the system when it introduced the K6.
Read more about this topic: Performance Rating