Your salary consists of a basic salary based on your seniority. When employed as a member of scientific staff (i.e. research assistant, PhD Fellow, postdoc, assistant professor, associate professor, professor, professor with special responsibilities) you will receive a centrally agreed supplement (tillæg) attached to the specific job category.
If you hold a professorship your salary is not based on seniority, but placed in a separate and higher salary bracket.
You can find more information about the job structure (including a description of the different job categories) in this memorandum from the Danish Ministry of Higher Education and Science:
Job Structure for Academic Staff at Universities