If you're making a comparison to another stock on a stock specific board for instance,using charts as an example to explain your comments in that the two stocks are similar in technical chart progression.
Or, the two stocks have the same company officers or PR provider or share the same office, something that ties the two together as a point of interest, should be alright in topic discussion on a stock specific board.
It's when the discussion varies off into areas that don't have any connection to the stock whatsoever, that posts become in jeopardy of being removed by moderators.