What is your performance rating scale? Does your organization have rating distribution guidelines?

1-5 rating scale, utilizing bell curve

Rating 1-5. And We use a Bell Curve to manage final outcomes

We are transitioning from a 5pt rating scale to a 4 rating system to force greater distinction.  We've found with an odd system that we end up with a pool in the middle we are looking to differentiate the high performers and believe this will allow us to do that.  At the same time we are implementing an assessment of what and how - employees will be evaluated against what they accomplished - goals and how they accomplished it (the Values) resulting in an overall performance rating.  There will be 4 overall performance ratings however because we are evaluating "what" and "how" we will be able to construct a calibration matrix for assessment similar to talent assessment.

We also have a 5pt rating scale, which we review twice per year. We assess a few different points: Communication & Feedback, Teamwork, Adaptability, Time management and Proactiveness, Problem Solving and Quality of Work, and Accountability.
We then cross this with another set that is a matrix of probability of churn (1-3) vs Impact of churn (Normal Employee, Important Domain knowledge loss, Risk of losing potential Rockstar, Important for future growing Business, Impact on team structure, OK to lose).
These 2 sets help us have a better understanding of where an employee is and how to manage them

We have a 5 pt rating scale (often misses objectives, Developing, Performs well consistently, exhbibits excellence, always exhibits excellence) with a distribution guidance of 5-5-60-20-10). We have a midyear with no ratings. Distribution is a guidance and not forced.