Any fat that goes in your mouth is a "dietary" fat, whether it's a fat all on its own (for instance, olive oil) or fat naturally present in food (for instance, the small percent of fat in your typical chicken breast).
I'm not quite sure what you mean, here, or where you read what you read. Essential fats, such as are found in fish oils, flax, hemp and various nut oils (you may have run into mention of them under their aliases of Omega 6 and Omega 3) should always be a part of the healthy diet. You should supplement with fish oil, and/or eat oily fish such as salmon, herring, and/or flaxseed, walnuts and other high-Omega nuts and seeds.
Or, possibly, you mean, "when trying to build mass, one should increase one's dietary fats, but not by eating more saturated fats." Saturated fats, in general, are those found in animal products. (The one exception is tropical oils like palm and coconut oil, which were once demonized. Research is now suggesting, however, that they don't act like animal saturated fats in the body, but may instead be beneficial in some ways.) Anyway -- butter? All saturated fat. Likewise full-fat dairy products, likewise fatty meats like bacon and ham. Saturated fat is not so good for your circulatory system (in other words, it may increase your chances of a heart attack, stroke, or other such incident) and may raise "bad" cholesterol.
In short, if that is what you mean, then if you are trying to build mass and one of the techniques you wish to use is to increase your dietary fat, then do not go crazy on the breakfast bacon; instead, increase your consumption of, for instance, nuts and nut butters and other higher-fat foods that don't contain a lot of saturated fat.
And, needless to say, don't go anywhere near trans-fats. These chemically-altered fats are of the Devil, and you can mostly spot them by the word "hydrogenated" or "partly hydrogenated" on a label. Don't be adding dollops of margarine to your bulking diet!
