There are lots of “cola” drinks, but if you tried to market “Coco Cola”, I think it would only take a megasecond for some attorneys in Atlanta to come swarming down on you.
The point is, that the makers of “Coco Cola” would intentionally be trying to siphon off customers of Coca Cola by confusing them.
Regarding Monster - OK, that might seem silly, but we all know Monster.com right? They are an online job hunting/marketing business. If I start a new website called Monsters.com and only sell masks for Halloween, there is no case - two different types of business and no chance of confusing a customer.
However, if I start a job seeking website called Munster or Monstor or Monstar - well, you could make a good case that I was trying to use the name to confuse people into thinking I was Monster.com and they would easily be able to shut down that new website.
Same thing goes for taglines like “Let it out!”
As long as that is a tag for a facial tissue product, they could sue another facial tissue product for using some variation of the tag line, trying to confuse the general public into thinking they were the same product.
But if you had a line of elastic belts and used the phrase, “Let it out!”, there would be no confusion as they are two different products.
It does get tricky with some catch phrases - my guess is “You deserve a break today…” from McDonalds might be pretty rock solid after all these years and a case could be made that any use of that phrase whatsoever, by any company, would make people think that product had something to do with McDonald’s corporation - even if it was a brake repair shop using "You deserve a brake today… This is when things go to trial. In this case, I think McDonalds would win, as it would be fairly easy to prove the catch phrase is easily identified as McDonalds’ by the general public.
Trademark attorneys do lots of work when registering names and logos and tag lines - they have to research and be certain they are not infringing on a current mark. They also have to work hard to keep their mark from being copied or used by other companies. There was an entire department in the film studio where I worked that did nothing but register trademarks, and stop people from trying to use their trademarks.
One job that was interesting - if a script came along, they would give it to a few attorneys who would pour over every name in the script; you had to be careful not to use some company name or logo that is actually a real company or similar logo. You had to make certain that your fictitious, lead character name of Senator Martin McAllen was not the real name of a Senator in some state. There were occasional slip ups and people would sue the film studio if they thought their name/company was being used. Rarely would they win, but if you had an “evil doctor” named Dr. Xacto in Pasadena in the film, you had better be damned sure there isn’t coincidentally really a Dr. Xacto in Pasadena or you’re screwed.