Danish legislation stipulates that sows be kept loose or in groups from weaning until farrowing except for 3 days during insemination in sow housing established after 2015. . . From 2035 all pig unit must comply with these requirements. See the legislation here.
The mating unit represents the first stage in the sow reproduction cycle, and it is used for insemination of gilts and sows. Boars are housed in pens with limited contact to gilts and sows. During insemination the boar might be used for stimulation of estrus in the females by moving in front of the mating stalls. Blocking gates are used to control the location of the boar in front of the females which produces a strong behavioral effect showing if they are ready for insemination. Therefore, free access stalls are commonly installed as a safe housing system in new mating units since it allows control of individual feed intake, sow body condition while protecting the animals which is essential to obtain a high productivity
The design of the mating unit might vary. Often, the design includes an area with free access stalls and a lowered free space area with drained flooring and bedding. Sows and gilts are normally kept in the mating unit until 4 weeks after insemination and then moved to the gestation unit. For inspiration check out which design that might best suit your housing project.