Performance Review

This is a regular process of syncing the company with the employee. So that he does not ask the question "how to ask for a salary increase", but understands exactly what expectations are from him, how he meets them and when there will be a raise.

We proceed from the task: to synchronize with the employee on expectations and its compliance + to close the basic need for fair remuneration.
Ideally, in this process, the employee receives a timely increase as they develop. To do this, there must be a way to unambiguously infer their current skill level and degree of contribution to the product.

Skills

For example, it can be a list of skills shared by a product. For each skill, there are examples of the observed manifestation at the Junior, Middle, and Senior levels. For each skill, you can clearly understand whether the employee shows it and, if so, at what level.
This matrix is filled in for each employee by himself, his colleagues, and his supervisor.
This matrix is a way to ensure fairness between employees.
It is also a way to retrospectively assess the development of an employee, just look at the values a quarter ago.

Performance

The degree of contribution to the product is the most incomprehensible area. An attempt to formalize the contribution measurement process may lead to the introduction of a KPI per line of code. After this, ABBYY and Microsoft abandoned the Performance review. All successful examples of Performance Review imply an individual approach to each employee.

Regularity

See, above about the matrix of competencies, I say "a quarter ago". This process should be regular. Only in this way will we remove the need for the employee to monitor the fairness of their remuneration. Here it is important not to overdo it with frequency, because the overhead of such a process for the company is growing.

Transparency

And this process should be as transparent as possible for everyone in the company. And even cooler-for everyone in the industry. For me, the ideal of transparency is Open Source. See how cool the guys from Avito described their processes on github:

Descriptions of grades - about competencies and soft skills for each grade.
As an outside observer, it would be interesting for me to read more about the hard skills levels and examples of observed behavior for competencies.

The Performance Review process is about the process that determines promotion by grades and salary.

Result:

First of all, the process should be regular and transparent. These two factors are the easiest to provide, and they already solve most of the problem.
It is more difficult to make the process objectively measure both skills and contribution. All the examples of processes I found included a large proportion of the evaluator's subjectivity.
A good feature of the process is to take this subjectivity into account and guarantee a fair assessment.

As always, welcome to the comments. If you know where else to read about Performance Review, and especially if you have such experience, write in the comments.