In the Federation, if a criminal is sentenced to more than ten years in prison, it means that they are a serious offender.
They must be detained in a special prison designed specifically for serious offenders.
If someone's sentence exceeds fifty years, there's no hope of them ever getting out; they will spend their entire life in prison.
In the judicial system of the Federation, imprisonment has no upper limit; there isn't a precisely achievable ceiling.
For example, a hundred years is the limit, even if someone ought to be sentenced to a hundred and twenty years, only a hundred are given because that is the limit.
There is none.
There is no such notion, so in the Federation, some people's sentences could be hundreds or even thousands of years long.
As for why they aren't sentenced to death, it might be because the jury or judges think their crimes are grave, but not severe enough to require their death, so they are given an exaggerated sentence instead.
