Skonummerutlysning is not an official term, nor particularly common in Swedish conversation, but it describes unfair recruitment practices which job-hunters may want to be aware of.
It literally translates as 'shoe size advert', and it refers to a job ad which includes highly specific requirements. Like someone's shoe size, they have no bearing on the ability of applicants to do the job.
It's used by unscrupulous employers who are complying with rules requiring them to advertise positions publicly, but already have someone in mind for the role and so write the ad targeted towards their chosen person.
The specific requirements might mean that the pre-selected person can be easily justified as the right fit, or to put off others from applying. It might be something like experience of a specific software of department which isn't really needed for the job. This difficulty in identifying these ads is part of the problem, because it could lead to job-seekers wasting their time applying for roles they have little chance being considered for.
Note: when talking about shoe size more generally, skostorlek is a much more common term than skonummer.
The term skonummerutlysning is most commonly used within academia and public sector industries, where it's a requirement for jobs to be advertised publicly. It doesn't necessarily mean this kind of bias is more common in these sectors, but hiring managers are less likely to have the option to simply offer the job to their chosen candidate without first advertising it.
It's no secret that many jobs in Sweden are found through job-seekers' personal networks, and that isn't always a bad thing. It's a fine line to draw, because you could argue that it's understandable for a recruiter to opt for the known quantity out of two otherwise equal candidates, especially if there is already a good working relationship.
But it can mean that certain groups end up shut out of employment, and a transparent recruitment process is important not only to give all candidates an equal shot, but to ensure that the right people end up chosen for the role.
Så kallade skonummerutlysningar är ett brott mot lagen
So-called shoe size adverts are breaking the law