As far as I remember, when the PS4 was introduced, the privacy politics stated that each SEN account could be linked to a maximum of 2 systems of the same specs, maximum. So if I wanted to play on a friend's house with an account that,has never been connected to a PS4, that PS4 would be embedded as a primary. And if the second you use is your main, should it break or anything then you would not be able to reuse that account on a warranty ps4.
All this does is restrict pirate business of selling accounts. And for the country I live in, I massively approve of account selling. So this is just bullshit at it's best when seen from my perspective.