D&D Beyond's Content Management feature allows DMs to control, to some extent, what content they share with their players.
For a long time, content sharing was all-or-nothing in D&D Beyond. When you had content sharing turned on for your campaign, you shared all your purchased content with your players.
However, D&D Beyond now has a "Content Management" feature, where DMs can turn specific books on and off, so that their players can only read the books they want them to be able to read. You can access this feature through the "Content Management" button on your campaign page when you are the DM.
This comes with some caveats, though. If a player owns a book themselves, then of course you cannot prevent them from reading it. Furthermore,
Blocking a source will hide compendium content to prevent players from
reading it unless they own it. No options or content will be removed
from other tools such as the character builder, character sheet, or
Regarding the option to purchase a book as "Compendium Content Only:" Purchasing a book as "Compendium Content Only" just means that the book is available to read in D&D Beyond, but the specific content (spells, magic items, classes, races, feats, etc.) from the book is not available for your D&D Beyond character sheets.