The code is attached to the individual Professions. As far as I know it is controlling the chance/likelihood of a 'work death' for that profession.
If you look throughout the Profession.rsc file, you will see that the Miner has 'float _years = 4;' -- the miner is the most 'dangerous' profession, the likelihood of a death to a miner is higher.
A Buillder, Forester and Hunter I think are next, with 'float _years = 5;'
The lower the number, the more likely a 'work death' will occur. Higher the numbers, the less likely.
The code 'flost _population = 100;' I would guess would be the required population at which a death could occur.
I'm sure @Tom Sawyer
or another person from this great community will have more insight into this.