There are two basic ways how a DM can give out xp and levels to his players: Either for each fight or other xp-worthy activity, or in some "milestone" system where reaching a certain point of the adventure gets the players to a predetermined level. Many of the official WotC campaign books are curiously imprecise about how they want level progression to work. But once you add up the xp for the fights in the chapters of the books, you quickly notice that even with lots of random encounters they don't add up to the levels required in the content. So although it often isn't explicitly stated, apparently the WotC adventures and campaigns use a milestone system.
But what really confuses and annoys me is that throughout Out of the Abyss there are sprinkled little paragraphs with a title of "xp awards". And those are nearly always just ridiculously tiny. For example in Out of the Abyss it is said that the players should be level 7 by the end of chapter 6 / the start of chapter 7. But in chapter 6 there are xp awards for "quests" of 50 xp. Seeing how a players needs 9,000 additional xp to get from level 6 to level 7, handing out 50 xp seems like some sort of cruel joke. What is the purpose of that? I would think that if you wanted to play Out of the Abyss with a level progression system based on experience points, you would need to hand out really large story xp awards in order for the players to keep up in level with the content.
But what really confuses and annoys me is that throughout Out of the Abyss there are sprinkled little paragraphs with a title of "xp awards". And those are nearly always just ridiculously tiny. For example in Out of the Abyss it is said that the players should be level 7 by the end of chapter 6 / the start of chapter 7. But in chapter 6 there are xp awards for "quests" of 50 xp. Seeing how a players needs 9,000 additional xp to get from level 6 to level 7, handing out 50 xp seems like some sort of cruel joke. What is the purpose of that? I would think that if you wanted to play Out of the Abyss with a level progression system based on experience points, you would need to hand out really large story xp awards in order for the players to keep up in level with the content.