We have gross minus "various debits for retirement, health care, etc" (fixed, a percentage of the gross) and then minus income taxes (that vary from person to person)
You have your gross salary - let's say, 100k.
From that, before taxes, is deducted the employee health insurance premiums (may be $0 to ~10k), employee retirement contributions (up to 19k), perhaps health savings account contributions (up to 7k), and possibly some others. Obviously, these can be quite substantial! But, at least, some of them are "optional" in that you are, for example, free to underfund your 401(k).
After that, federal and state income taxes are taken out, and a special tax for Medicare and Social Security.
Many of these are impacted by the vagaries of each employer. If you know your pre-tax income, you can easily calculate your taxes. But figuring out what your pre-tax income will be is not something you can figure out from the headline gross number. The biggest factor is the employer health insurance plans and your expected premium contribution - getting this out of an employer before being hired is like pulling teeth.
Although HSA and 401(k) contributions are elective, so you'd think you could determine that ahead of time...well, if the employer doesn't have a HDHP, then you don't get an HSA at all. Conversely, if they do, the amount of the deductible and the amount of the possible employer contribution (which counts toward the maximum allowable contribution by the IRS) factor into how much you expect to come out of your pay check. This is also tough info to get out ahead of time. 401(k) contributions fall into a similar but less severe problem - there's often an employer contribution, but it counts toward the allowed maximum, so you cannot determine from your gross salary alone how much you will be contributing toward your 401(k), even if you know ahead of time the dollar amount you want to contribute. Employers are often a little better about telling you their 401(k) contribution benefits ahead of hiring, though.
You have the gross that is the usual negotiating point because the number sounds larger.
Then there are compulsory deductions (national, the amount decided by the law). They cover health insurance, disability, retirement, ... The amount deducted from your salary is also (for the most part) paid by the employer (same amount).
To this may be added extra money, again regulated by the law which is a percentage of your "gross before taxes" (above) - this is dependent on the size of the company, inflation, etc. (intéressement et participation)
We are now at "gross before income taxes". These taxes change between people (married, children, some specific renovations in your house, ...) but for the vast, vast majority of people (probably 95%) they are pre-calculated and you go to the national tax site, click next a few times and you are done (literally 3 minutes). The taxes themselves are deducted by your employer.
So what you get in your bank account is really a net amount you can spend. It is substantially smaller than the gross (about 50% I guess) and it would make much more sense to negotiate on that one because thi sis what you can actually spend.
