It is a measure of skill. Senior is when you are good enough to lead a team with your technical skills, staff is when you can lead a team of seniors. Typical people reach senior level after 5 years and never reach staff level, so a typical engineer would still be senior even after 20 years.
It is a bit silly to label those senior, but that is how the industry decided to label things. If your company is more strict then it just make their engineers look worse than they are, it is a great way to make engineers less likely to get poached but it isn't great for those engineers since it will look like people with 20 years of experience and great track record stagnated after 5.
Spot on, maybe just an explanation of what "skill" means here: it's not just technical skills, beyond the "lowest terminal level" political skills matter more and more.
It is a bit silly to label those senior, but that is how the industry decided to label things. If your company is more strict then it just make their engineers look worse than they are, it is a great way to make engineers less likely to get poached but it isn't great for those engineers since it will look like people with 20 years of experience and great track record stagnated after 5.