Yeah, so essentially they use 100% beef, just like everyone else does, and then puts seasoning and other shit in it...just like everyone else does. The only difference is that Taco Bell mixes it all up, so they stupidly labeled it as 88% beef for some reason (I'm assuming that moron got fired). That's like saying a Wendy's spicy chicken is only 88% chicken, because all the spices and breading and shit isn't chicken. If, for some reason, Wendy's sold a "ground spicy chicken" taco, they might call it 88% chicken too. If they were dumb.
It is still 100% beef, just like everyone else. They just included the additives for some stupid reason.