#ads.txt file denotes what networks are allowed to sell ads to be published on our site - in this case it is none, and the way to specify none is by using this 'placeholder' line, see page 6 https://iabtechlab.com/wp-content/uploads/2019/03/IAB-OpenRTB-Ads.txt-Public-Spec-1.0.2.pdf
#we also serve the same file at app-ads.txt
placeholder.example.com, placeholder, DIRECT, placeholder