Don't bump threads with textposts, it takes no effort to attach an image.
There is no basis for it, it depends on the mod/janitor that is around at the time. Sometimes it gets deleted others not. Moderation on /c/ can be non-existent at times which causes more things to slip through. It not worth it to even really discuss the topic since it hasn't seem to have changed over the last couple years.
Context is very important, not all underboob is lewd but that doesn't mean all underboob can be posted. Depends on the artist and the image question, sometimes its great art and other times is the artist being a horny faggot. And just because one might think something isn't lewd, doesnt mean it actually isn't lewd.