As per forum #5017, as far as I've understood this tag is/was supposed to counteract and balance the yuri tag - As in, "if it doesn't fit under the yuri tag definition, tag it with 2girls instead".
Example posts of this would be stuff like post #256934, post #257067, and post #2645. Example posts of what it should not be would be post #257559, post #257657 (?), and post #256370. Basically "If it isn't at least mildly suggestive of a relationship, it's not yuri, and thus should fall under 2girls instead". Am I right about this, or completely out in the blue?
If this is how it's supposed to be, the tag should be cleaned up and a more explenative wiki entry written, but I didn't want to do that yet since I wasn't completely sure this is indeed the case.
Updated by LaC