The basic use of meal replacements is to substitute a meal. When one can't eat solid food you can use a replacement pack/protein shake to do the job. Once you understand the importance of eating 5 to 6 meals daily and is following a meal plan to help you gain weight. There will come a time when you can't or don't have the time to prepare a meal, then you can use a meal replacement bar or shake. I would recommend at most two meals replacement packs out of six meals.
There are also meal replacement diets where you tend to drink shakes that will help you lose weight. Those are low in calories and low in fat. Each Packet contains around 200 calories, 2 grams of sugar and a very low fat percentage. The most recommended one is MyOplex Lite from EAS.
And there are those that will help you to gain weight.
What Are Meals Replacement Packs (MRP's)
They contain the right amounts of carbohydrates, vitamins, minerals and protein for optimum nutrition. They can be used as post workout meals or in between snacks. You can purchase meals replacement powders, bars or packs. These packs are more suitable because they come in boxes with 24 or 30 packs and are easy to use and mix. No measuring just open one pack and blend.
For weight gain you should seek packs with 30 or more grams of protein, the well know brand is MyOplex Deluxe from EAS which contains 29 vitamins and minerals and at least 300 calories.
For fast weight gain I would eat four meals daily and use two replacement packs.
The main reason for this is that consuming so much food for a thin person is very hard. You actually get tired of eating, and that is why it is easier to drink a couple shakes.
Another big benefit is the cost. They are not expensive and taste really good.
Important Tip: Meal planning is important and I would recommend using protein shakes only for a short period of time. Three months of usage is more than enough. Once you reach your required weight gain then I would stop using these shakes and supplement with healthy snacks to keep a balanced weight.