Tattoo

I extremely doubt thats the purpose, since Snake has been in VR several times and he's tattoo-free.

Raiden's tattoos could be from his days in war, the Liberian Civil War, in which he was a child soldier. Although his age might conflict with him getting tattoos.

The Liberian War started in 1989 and ended in 1996. Raiden claimed that he was only six when he first held an AK, and at the age of ten he became the leader of the Small Boy Unit.

Since the war only lasted seven years, and Raiden was in the war for at least four years since it began. Solidus says "The 80s, the Civil War, you were one of the best who fought in that conflict". I assume this means that Raiden was in the war in 1989, and saw it through to the end. SO he was probably six when it started, and 13 when it ended and taken to the US for a "new life". Probably an orphanage, since Raiden claims to never had a family.

So if Raiden was six in 1989, he was born in 1983. He would be 26 in MGS2 - which is acceptable based on his appearance.

Getting back on topic, Raiden would have gotten tattoos between the age of 6-13. Giving a 13 year old tattoos seems more acceptable because he's at least a teenger by then.

But, ofcourse, it may not even be from the war, but a sort of "branding" by The Patriots. I mean, Raiden didn't even know that his real blood was replaced by Artifical Blood before the mission, meaning while being "prepared" he was not aware. But the tattoos don't look fresh, so, he would've gotten them before that.

How would The Patriots give him tattoos without screwing up their plan, since he's not allowed to know they exist? Well, it's possible!

Solidus reveals that Raiden has Cerebal Implants, and that they were responsible for altering his memory. Meaning Raiden could have been "branded" with tattoos, but because of the implants, had no idea of the origin of the tattoos, and just accepted that they're there.

The reason I think they are a sort of "branding" is because if you notice on Liquid's arm that is surgically attached to Ocelot, he has a barcode tattoo near the wrist, I believe. Liquid didn't have that tattoo before his death. And since Otacon and Solid stole his body from "some organization", which seems to connect to The Patriots, it seems to be kinda their trademark.

Also, with the bar-codes, on Raiden's left side of his chest, over his heart, are the numbers something like 34-38 or something, kinda hard to tell/remember. It isn't a bar-code, its a number.

But, again, it could simply be tattoos that Hideo thought looked cool. Psycho Mantis had bar-code tattoos as well.