Staž (having a ž - zh - it must be of french origin) being the time-on-work, what counts for retirement, or time in the firm. In medicine, that's internship.
There are few kinds of it, depending on where it counts. „Staž u firmi“ - in-company, is the length of employment in it. „Radni staž“ - the active, aka work-, is the number of years, months, days of regular work. Regular means on-the-books, taxes and contributions paid, so it's the accumulated work which is calculated when retiring.
Then there's stažiranje, 'staging', which is the (often mandatory) year or two that a fresh graduate must spend on the job, often being rotated from division to division, to learn the ropes. Applies to doctors, military officers and few other cases where the responsibility and complexity of the job are above the textbook level.
A stažer is, then, anyone who's on stažiranje - be it a pilot, doctor, petty officer.
The trouble with retirement and staž was that, while the early versions of the SFRY - namely FDY and then FPRY, promised full employment for everyone, there was also the assumption that the retirement will require full 40 years (35 for women) of work, day to day. So anyone switching jobs and getting caught by some administrative snafu, or after 1965 even real unemployment, would have a hole in there. Which was then a problem. Some solved that by staying longer at work, and in the latter years some time could be bought off - if, for example, a few months were missing etc. This was called 'incontinuous staž' and 'buyoff of staž'. The sum to pay was a pittance, compared to what one would lose by the reduction in retirement pay which would seriously lower the amount. Just missing three months would lower it by 2-5%, the percentage getting more severe as the years went on and the country (SFRY, then FRY, then Ser&Mo, then Serbia) slid deeper into the neo-something capitalism.
The cause for missing months was more and more not just simple unemployment, but also various cheats by employers (including the government itself), where they'd keep you on salary but skipped the payments for social services. Often people ended with 20-30% less retirement, because they couldn't get the employer to pay for it, even belatedly, because the employer was now abroad (in, say, Slovenia or Montenegro), dissolved, imprisoned and whatnot.