They are not tattoos as such. According to Ridley Scott quoted in Future Noir, it was a half-developed idea they had - another thing inspired by Jean Giraud. What would have been interesting was having the Replicants "built up" from parts. (This wouldn't have worked with other back-story that was developed though.) And it would have meant too long in makeup for Rutger anyway.

So they then thought of markings that denoted points where, say, a space war suit would plug in to sockets. But even that idea got left behind and we were left with markings that are simply "A curious detail." So it is really up to you how you would like to interpret them.

The markings were applied by Freddy Blau of Reel Creations through a process of silk-screening them on and then hand-finishing them. It took four hours to do!