Simple enough. It used to be a tag that had quite a number of pictures it could apply to, before Riversyde inexplicably aliased it and the other implied_* tags to invalid_tag.

For an example of a post where implied_vore would be valid, see post #972678. The laptop visible to the viewer has a list of donation goals for making that Nidoqueen fatter, with her eating that stuff when the goal was reached. One of the goals was 'the pizza boy'. Therefore, vore is implied to have happened.