There are different ways you can use feeds in the editor. For example, a dropdown of items to choose from or a search bar to search for items in your feed. You can read more about the different options that are available here
If you don't use a search helper you can have parameter fields and depending on what you type or paste in, this will pull in the relevant content to your emails. An example of this is if you have a product database and you paste in a product ID into the field in your module.
When using feeds in this way, there will be a 'fetch all' button in Document settings
This will look through all the modules using a feed in that email and pull in the latest content based on the parameters that have been used.
This option is also available when you use a search helper and it only returns one option to choose from. This is so you don't have to go through each module individually and pull in content, you can do it all in one go for each email.