As Nik tools only work with ‘developped’ pics (leave FilmPack Elite aside), PL converts the raw-file into tiff-format (leave jpg-format aside) and sends it over to Nik (external program / code) to be further processed.
Used as PlugIn for PS or Affinty Photo, the Nik tool (external program / code) does the very same, but the so changed tiff-file now results straight away in a (new) layer of the said pixel editor – allowing further manipulations like masking, varying the layers opacity etc.
Different to AP – PS also allows the Nik tools to be handled as “smart filters”, that is the processed file (layer) now can be fully re-edited in the Nik PlugIn without starting from scratch.
This functionality goes beyond the ‘double layer’ mode when exported from PL to Nik, where one can only re-edit the last used Nik filter.
Due to different coding, the (so far) new Nik Collection 4 modules (VIV3, SEP3) can’t be used with PS CS anymore → https://support.dxo.com/hc/en-us/articles/4409304781201, which is really unfortun.