Selection ratio refers to the ratio of the number job positions to the number of job applicants and is used in the context of selection and recruitment

Recruitment

Recruitment refers to the process of attracting, screening, and selecting qualified people for a job. For some components of the recruitment process, mid- and large-size organizations often retain professional recruiters or outsource some of the process to recruitment agencies.The recruitment...

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It is typically assumed to be a number between 0 and 1 where a number closer to zero implies that there are many applicants for any one position.

The selection ratio provides information about the value of assessment tools, such as interviews, work samples, and psychometric tests.
When the selection ratio is close to one, most applicants will need to be hired in order to fill the available positions.
As such knowing that one applicant is likely to perform better than another is of limited value.
In contrast when the selection ratio is closer to zero, assessment tools have greater value in indicating which subset of job applicants are likely to perform best.
Estimates of the selection ratio can form part of estimates of utility

Utility

In economics, utility is a measure of customer satisfaction, referring to the total satisfaction received by a consumer from consuming a good or service....

for a given selection and recruitment system.